Free space optical (FSO) laser communication system employing fade mitigation measures based on laser beam speckle tracking and locking principles

ABSTRACT

A free-space adaptive optical laser communication system having signal transmission and reception channels at all terminals in the communication system, wherein wavefront sensing and wavefront correction mechanisms are employed along signal transmission and reception channels of all terminals in the communication system (i.e. adaptive optics) to improve the condition of the laser beam at the receiver (i.e. reduce the size of the spot a the detector plane). Speckle-to-receiver-aperture tracking mechanisms are employed in the transmission channel of the communication system and laser beam speckle tracking mechanism in the reception channels thereof, so as to achieve a first level of optical signal intensity stabilization at signal detector of each receiving channel. Speckle-to-fiber/detector locking mechanisms are also employed in signal receiving channels of all terminals in the communication system so as to achieve a second level of optical signal intensity stabilization at signal detector of each receiving channel.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an improved bi-directional lasercommunications link between two optical transceiver platforms (e.g. anairborne platform and a satellite) having a way of and a means fordynamically stabilizing variations or fluctuations in the intensity ofdetected optical carrier signals (i.e. suppressing fade events) that arecaused by atmospheric effects such as turbulence, which tends to degradelink performance. Such fade compensation is particularly important forfree-space optical laser communication systems requiring thetransmission over of minimum laser beam power levels over fiber-to-fiberfree space optical communication links.

2. Brief Description of the State of Knowledge in the Art

There is great interest in super-broad band free-space optical (FSO)laser communication systems as they are capable of securelycommunicating information at high data rates in point-to-point andmulti-point-to-multi-point communication networks.

In military applications, such laser communication systems and networksoffer a level of superiority and security over radio-frequency (RF)based communication system which have relatively limited band-widths,and thus data transfer rates, as well as being susceptible to RF-basedjamming techniques intended to interfere and disrupt the performance ofsuch systems. In commercial applications, such laser communicationsystems can be rapidly installed in point-to-point andmulti-point-to-multi-point configurations (using buildings and towers assupport structures for such laser communication platforms) at asignificantly reduced expense in comparison with micro-wave-basedsatellite communication systems. Examples of FSO laser communicationssystems are disclosed in U.S. Pat. Nos. 6,657,783; 6,643,467; 6,348,986;6,347,001; 6,314,163; 6,285,481; 6,286,944; 6,181,450; 6,151,340;6,122,084; 5,844,705; 5,786,923; 5,786,923; 5,754,323; 5,710,652;5,606,444; and WIPO Publication No. WO 03/003618, each said prior artreference being incorporated herein by reference.

However, FSO laser communication systems are not without challenges andproblems.

In particular, the free-space optical communication links in FSO lasercommunications suffer from atmospheric conditions such as turbulence andthe like which cause aberrations in the spatial phase of the wavefrontof the modulated carrier laser beams as such carrier laser beams aretransmitted between terminals in such communication systems. Dependingon the distances between the laser communication terminals, such spatialphase aberrations can evolve into spatial intensity aberrations in thelaser beam received at the entrance pupil of the receiver module of suchterminals, degrading the bit error rate (BER) of such communicationsystems.

Studies have been conducted by Adaptive Optics Associates, Inc. ofCambridge, Mass. to quantify these wavefront errors over the range ofpossible link geometries. The uplink and downlink have been consideredseparately because the atmosphere affects the propagation differentlydepending on its direction. The density of the atmosphere declinesrapidly with altitude and thus most of the wavefront errors aregenerated close to the aircraft. Integration of the analytical model ofthe atmosphere proposed by Fried (JOSA, 56,1380, 1966) shows that, forthe altitudes of interest in this study, the effective distance of theturbulence from the aircraft is about 3 km.

For the light propagating up to the aircraft the wavefront errors pickedup close to the aircraft evolve along the long path to the satellite sothat the beam that reaches the satellite has both phase and intensityerrors. This behavior is described by the eikonal equation (Born andWolff, Principles of Optics p. 112), which shows that the change inintensity between two planes perpendicular to the propagation directionis proportional to the product of the curvature of the wavefront and thepropagation distance. This effect is the source of scintillation in thebeam.

For light coming down to the aircraft, the situation is very different.The wavefront has only 3 km to evolve before reaching the receiver andthus is almost purely a phase error without any scintillation.Therefore, the airborne to spaceborne direction is the most problematicdirection in this link. This fundamental difference leads to verydifferent requirements for the satellite and aircraft transceivercompensation systems.

Modeling of Atmospheric Turbulence

To assess the impact of atmospheric turbulence on the communicationslink, a model of the atmosphere has been developed. This model is basedupon one of the standard models (e.g. Clear 1 model developed for theABL program) supplemented with field data from the ABL-EX experiment,data from astronomical observatories (Masciadri, et al. Astron.Astrophys. Suppl. Ser. 137,185-202, 1999.), and the AFRL 3D modelingproject(http://www.hpcmo.hpc.mil/Htdocs/UGC/UGC00/present/frank_ruggiero_pr.pdf).Of particular importance in this study is the modeling of the turbulentlayer associated with the tropopause, determination of the range ofaltitude for the tropopause, and modeling of the effects of a jetstream. This is because, unlike an observatory that can be sited at alocation known to have very low turbulence, the communications link mustoperate even if the aircraft is below the tropopause and a strong jetstream is traversed at some zenith angles.

Using the atmospheric model, turbulence profiles along various linkpaths have been determined. The important parameters are the altitude ofthe aircraft, the velocity of the aircraft, and the altitude and orbitalpath of the satellite. The aircraft altitude determines the startingpoint of the link path. The aircraft velocity sets the rate at which thebeam translates through the atmosphere. The satellite altitude and pathdetermine the apparent slew rate of the beam. FIG. 1 describes theseplatform parameters. Once a set of link paths is defined from theseparameters, the atmospheric model has been used to create a series ofphase screens at different altitudes. These are inputs to the AOA WaveOptic Propagation Code that is used to calculate the effects on thelaser beam. Such modeling has been performed for both the uplink anddownlink directions. One additional input to the Wave Optic PropagationCode is the aero-optical effect of the flow field around the window ofthe transceiver on the aircraft. This is modeled as a time varying phasescreen. The results of this simulation will be a time sequence of phaseand intensity maps for the entrance apertures of the two receivers.

FIG. 1B shows a typical phase map calculated for a horizontalcommunications link with strong turbulence. FIG. 1C shows a timesequence of detected laser beam spot intensity distributions calculatedusing a simple model atmosphere for propagation to a satellite at 1000km altitude from an aircraft at 35,000 feet. The bar in each framerepresents 10 m at the satellite. The second through fourth frames inthis time sequence show a dark (rather than bright) speckle of severalmeters extent. Such an event would lead to a deep fade of the linksignal for any receiver aperture of less than a few meters diameter.

The atmospheric model described above is used to examine other aspectsof the laser communication link compensation problem. For example,because of the long path to the satellite, the light travel time to thesatellite becomes significant. The result is that the transmitted beammust be pointed ahead of the apparent location of the satellite, just asa hunter must point his gun ahead of a flying duck to successfully hitit. This point-ahead angle is typically tens of microradians.Difficulties arise when this point-ahead angle becomes larger than theisoplanatic angle of the atmospheric turbulence. This is the angle overwhich the aberration is correlated. If the point-ahead angle is largerthan the isoplanatic angle, the wavefront error measured from a sourceco-located with the satellite does not correctly represent theaberrations that will be experienced along the path of the transmittedbeam. Also, as wavefront tilt is the strongest atmospheric aberration,this aniosplanatic effect can lead to serious pointing errors in thetransmitted laser beam if the employed laser beam tracking algorithmuses only the satellite apparent position to estimate the error. A greatbody of research exists on this effect, both in the astronomicalliterature and from HEL DEW projects. If tilt anisoplanatism appears tobe a significant problem for a given application, then more advancedtracking algorithms need to be drawn from this prior research and testedin the simulation.

Using the above-described techniques, some initial modeling has beenperformed to verify that atmospheric turbulence is a significant issuefor aircraft to space optical communications links. A model atmospherewas constructed based on the analytical model of Fried (JOSA, 56,1380,1966). The Fried model was modified to include the observed “bump” inturbulence in the region of the tropopause and the strongly layeredstructure of turbulence. FIG. 1D shows a typical C_(n) ² profilegenerated by this model. This model was used to estimate the integratedturbulence along a vertical path from aircraft at various altitudes. Theresults for one realization of turbulence are shown in FIG. 1E. Fromthese integrated turbulence estimates pertinent parameters for thedesign of a compensation system were derived. These included theatmospheric coherence length (Fried's r_(o) parameter) and the coherenceangle, known as the isoplanatic angle. These calculations were madeusing a wavelength of 1550 nm.

FIGS. 1F1 and 1F2 show some typical results derived from modeling thisphenomenon. In particular, FIG. 1F1 shows that for altitudes below thetropopause (about 45,000′ in this case) the coherence length iscomparable to, or smaller than potential transceiver aperture sizes.This means that significant atmospheric aberrations will occur at thesealtitudes. FIG. 1F2 shows that at those same altitudes, the isoplanaticangle is in the range of tens of microradians. Since this is comparableto the point-ahead angle required for a link to a LEO satellite, theissue of tilt anisoplanatism must be treated carefully in any particularapplication.

Wave-Optic Propagation Simulations

To further assess the impact of this level of turbulence on a free spaceoptical communication link, some simple wave-optic propagationsimulations were performed. Using the integrated C_(n) ² value, a singlephase screen was generated for an aircraft altitude of 35,000 feet. FIG.1G shows a pseudo-color representation of such a phase screen. The fullextent of the screen is 1 m×1 m. Subsections of this phase screen wereextracted along a path to simulate the motion of the aircraft past theturbulence. In this simple model, the turbulence itself is frozen intime. The extracted phase screen was then applied to a uniformlyilluminated circular pupil of 150 mm diameter and the resultingwavefront propagated to a range of 100 km.

While the use of a single phase screen does not model the realthree-dimensional distribution of the turbulence, because theatmospheric density falls off fairly rapidly with height, it is adequatefor obtaining a rough idea of the effects of turbulence on the beampattern at the satellite. FIG. 1H shows two examples of the beam profileat the satellite compared with the diffraction limited profile (leftimage). Each image covers about an area 100 m by 100 m. In the case ofthe center beam profile, little power is lost due to the aberration.However, the laser beam profile on the right shows that significant fadeevents can occur as the receiver aperture could easily lie within one ofthe dark (speckle) regions of the laser beam spot image.

A sequence of 576 beam profiles was generated during wave opticpropagation simulation. For a platform velocity of 200 m/s this sequencerepresents about 0.1 s of flight time. The temporal variation of thereceived power was calculated for this sequence and is shown in FIG. 11.There is one fade event of about 10 db in this data set. Thus, in anylink compensation system, it will be necessary to determine how well itsuppresses these occasional fade events in addition to the expectedimprovement in link power due to the improved beam pattern.

The effect on laser beam pointing has also been examined. For eachprofile, the location of the centroid of the light was calculated. Therms variation in pointing direction was found to be 12.6 μrad with apeak deviation of 28.7 μrad. This rms variation in pointing is slightlylarger than the diffraction limited beam radius and thus must becorrected to maintain an adequate communication link signal.

To determine the parameters for a compensation system, the spatialfrequency content of the phase aberrations must be examined. Each of theextracted phase screens is decomposed into Zernike polynomials and thetemporal intensity fluctuations of the Zernike coefficients determined.FIG. 1J displays the rms variation in each of the lowest 15 Zerniketerms. Notably, the bulk of the aberration is contained in the firstthree terms, the two tilt terms and defocus. That is typical of allatmospheric aberrations. Correction of just these three terms, intransmission and reception channels of the system, would produce a laserbeam that, in a statistical sense, would have a power density close tothe diffraction limit. For typical imaging or directed energyapplications, this is the figure of merit used to assess the degree ofcompensation to be achieved. Also, as turbulence is a random process,there will be occasions when higher order aberrations are strong enoughto further disrupt the laser beam profile.

Traditional Adaptive Optics (AO) Atmospheric Compensation

One technique that has been used to mitigate the fading problem in freespace optical (FSO) laser communication links, is traditional adaptiveoptics (AO) atmospheric compensation. In principle, an AO compensationsystem on the transmitter can improve the condition of the beam at thereceiver and help to reduce the fluctuation in received power. Anadaptive optics compensation system on the receiver should also reducethe size of the spot at the detector plane.

Unfortunately, under conditions like those of the Wave-Optic PropagationSimulations described above, the performance of phase only AOcompensation systems is very limited. The reason lies in the differencein the strength and distribution of the turbulence along a horizontalpath compared to the vertical path of typical atmospheric AOcompensation systems (e.g. astronomical adaptive optics). Withturbulence roughly uniformly distributed along the path, phase errorsclose to the transmitter evolve into strong intensity variations at thefar end of the path. Similarly, the light from the beacon on thereceiver that is used as the source for the transmitter AO compensationsystem arrives at the transmitter with strong intensity variations.Portions of the received beam with very low intensity lead to erroneousphase measurements and corrupt the calculated wavefront. Even if thewavefront were measured perfectly and the correct phase compensationwere applied to the outgoing laser beam, there would still be intensityvariations across the laser beam at the receiver. This is because thetransmitted laser beam is typically of uniform intensity while the laserbeam that would produce uniform intensity at the receiver has intensityvariations that mimic those of the laser beam from the beacon. This canbe understood quite simply. Consider a portion of the laser beam fromthe beacon that is of low intensity. The light in that part of the laserbeam has been diverged by the atmosphere to other parts of the beam.When the uniform transmitted beam passes through that same path, thelight is converged by the atmosphere to produce a region of highintensity. What is needed to properly compensate the transmitter is fullconjugation of the complex electric field of the laser beam from thebeacon. While techniques exist for doing this, none has beendemonstrated in the field.

At the receiver end, the compensation problem is not as daunting. Thereare the same problems due to regions of low intensity in the receiverpupil mentioned with regard to the transmit side of an AO compensationsystem. However, if the phase is correctly measured and compensated,then the laser beam spot at the detector should approach a diffractionlimited size. This compensation system cannot do anything to reduce thesize of variations in the power coming into the receiver. If, due tothose variations, no light enters the receiver's entrance pupil, then noamount of phase compensation will increase the received signal power.

On top of these basic physical limitations of AO compensation systemsfor horizontal paths, there are also large and costly engineeringproblems due to the high temporal frequency characteristics ofatmospheric turbulence. Measurements over a 16 km horizontal path showsignificant power in the full aperture tilt at temporal frequenciesabove 1 kHz. Higher order aberrations probably include even highertemporal frequencies. This requires an AO compensation system with acorrection bandwidth of at least 1 kHz. Such response characteristicsare beyond the performance of any existing AO systems.

In summary, optical communications technology has rapidly advanced andis at the point where communication systems via fiber at 40 Gbs arecommercially available. There are applications that prevent the use of afiber connection, such as those involving moving platforms.

However, the transition of the light from fiber to free space presentsproblems. The achievement of very high data rates requires the use ofsingle mode fiber to prevent mode dispersion from corrupting the datastream. Conversion of the light signals to electrical signals at veryhigh rates places constraints on the physical size of theelectro-optical sensor. The capacity of a detector is proportional toits area and, because of its inherent RC time constant, larger detectorsare necessarily slower. For these reasons, to couple free spacepropagating light into a fiber or onto a detector requires that thelight be concentrated into a very small region.

In the case of a single mode fiber, the mode diameter is similar to thediffraction limited spot size defined by the numerical aperture (NA) ofthe fiber. For efficient coupling, the NA of the coupling lens mustmatch the NA of the fiber. This means that the focal spot needs to beclose to diffraction limited to properly couple the light from freespace to the fiber.

Unfortunately, propagation of light through the atmosphere introducesaberrations in the optical wavefront that prevent reaching thediffraction limit. To some extent, these aberrations can be correctedwith adaptive optics. However, under conditions of strong turbulence(such as encountered in a free space link close to the ground), thedisturbance of the optical beam include both phase and intensitycomponents. Traditional phase only adaptive optics cannot compensate forthese intensity disturbances. The result is that FSO links under theseconditions experience deep signal fade events that lead to unacceptableinterruptions in the communications link.

Therefore, there is a great need in the art for an improved method andapparatus for mitigating or compensating for the effects of lasercarrier signal fluctuations (i.e. fading) detected at the receiver ofFSO laser communication systems, while avoiding the shortcomings anddrawbacks of prior art methodologies and apparatus.

OBJECT AND SUMMARY OF THE PRESENT INVENTION

Accordingly, it is a primary object of the present invention to providean improved method of and apparatus for mitigating or compensating forthe effects of laser carrier signal fluctuations (i.e. fading) observedat the receiver of FSO laser communication systems, while avoiding theshortcomings and drawbacks of prior art methodologies and apparatus.

Another object of the present invention is to provide a free-spaceoptical laser communication system supporting optically-separated signaltransmission and reception channels, and employing laser beam speckletracking mechanisms and speckle-to-fiber/detector locking mechanismsalong the signal reception channels thereof for automaticallystabilizing variations in the detected intensity of received laser beamcarrier signals caused by atmospheric turbulence along said signalchannels.

Another object of the present invention is to provide a free-spaceoptical laser communication system having optically-separated signaltransmission and reception channels at all terminal points in thecommunication system, wherein laser beam speckle tracking (i.e.following) mechanisms are employed in reception channels of the systemto achieve a first level of optical signal intensity stabilization atsignal detector of signal reception channel; and whereinspeckle-to-fiber/detector locking mechanisms are employed along thesignal reception channels of system to achieve a second level of opticalsignal intensity stabilization at the signal detector in the signalreception channel.

Another object of the present invention is to provide a method of andapparatus for automatically stabilizing variations in the intensity ofreceived laser beam carrier signals caused by atmospheric turbulencealong the signal reception channels of a free-space optical lasercommunication system supporting optically-separated signal transmissionand reception channels.

Another object of the present invention is to provide a free-spaceoptical laser communication system supporting optically-combined signaltransmission and reception channels, and employing laser beam speckletracking mechanisms and speckle-to-fiber/detector locking mechanismsalong the signal reception channels thereof for automaticallystabilizing variations in the intensity of received laser beam carriersignals caused by atmospheric turbulence along said signal channels.

Another object of the present invention is to provide a free-spaceoptical laser communication system having optically-combined signaltransmission and reception channels at all terminals in thecommunication system, wherein laser beam speckle tracking mechanism isemployed in the signal reception channels of all terminals in thecommunication system to achieve a first level of optical signalintensity stabilization at signal detector in each signal receivingchannel, and wherein speckle-to-fiber/detector locking mechanism areemployed in the signal reception channels of all terminals in thecommunication system to achieve a second level of optical signalintensity stabilization at signal detector in each signal receptionchannel.

Another object of the present invention is to provide a method of andapparatus for automatically stabilizing variations in the intensity ofreceived laser beam carrier signals caused by atmospheric turbulencealong the signal reception channels of a free-space optical lasercommunication system supporting optically-combined signal transmissionand reception channels.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system supportingoptically-separated signal transmission and reception channels andemploying laser beam speckle tracking mechanisms andspeckle-to-fiber/detector locking mechanisms along the signal receptionchannels thereof for automatically stabilizing variations in theintensity of received laser beam carrier signals caused by atmosphericturbulence along said signal channels.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system having optically-separatedsignal transmission and reception channels at all terminals in thecommunication system, wherein wavefront sensing (WFS) and wavefrontcorrection (WFC) mechanisms are employed in the signal transmission andreception channels of system (i.e. traditional adaptive optics) toimprove the condition of the laser beam at the signal detector in thesignal reception channel (i.e. reduce the size of the spot at thedetector plane); wherein laser beam speckle tracking mechanisms areemployed in the transmission channels of system so as to achieve a firstlevel of optical signal intensity stabilization at signal detector inthe signal reception channel; and wherein speckle-to-fiber/detectorlocking mechanisms are in signal reception channels of system to achievea second level of optical signal intensity stabilization at signaldetector in each signal reception channel.

Another object of the present invention is to provide a method of andapparatus for automatically stabilizing variations in the intensity ofreceived laser beam carrier signals caused by atmospheric turbulencealong the signal reception channels of a free-space adaptive opticallaser communication system supporting optically-separated signaltransmission and reception channels.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system supportingoptically-combined signal transmission and reception channels andemploying laser beam speckle tracking mechanism andspeckle-to-fiber/detector locking mechanisms along the signal receptionchannels thereof for automatically stabilizing variations in theintensity of received laser beam carrier signals caused by atmosphericturbulence along said signal channels.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system having optically-combinedsignal transmisssion and reception channels at all terminals in thecommunication system, wherein wavefront sensing and wavefront correctionmechanisms are employed in signal transmission and reception channels ofall terminals in the communication system (i.e. adaptive optics) toimprove the condition of the laser beam at the receiver (i.e. reduce thesize of the spot a the detector plane); wherein laser beam speckletracking mechanisms are employed in signal reception channels of allterminals in the communication system to achieve a first level ofoptical signal intensity stabilization at signal detector of eachreceiving channel; and wherein speckle-to-fiber/detector lockingmechanisms are employed in signal receiving channels of all terminals inthe communication system to achieve a second level of optical signalintensity stabilization at signal detector of each receiving channel.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system supportingoptically-combined signal transmission and reception channels andemploying a speckle-to-receiver-aperture locking mechanism along thesignal transmission channels of the system and laser beam speckletracking mechanisms and speckle-to-fiber/detector locking mechanismsalong the signal reception channels thereof for automaticallystabilizing variations in the intensity of received laser beam carriersignals caused by atmospheric turbulence along said signal channels.

Another object of the present invention is to provide a free-spaceadaptive optical laser communication system having optically-combinedsignal transmission and reception channels at all terminals in thecommunication system; wherein wavefront sensing and wavefront correctionmechanisms are signal transmission and reception channels of allterminals in the communication system (i.e. adaptive optics) to improvethe condition of the laser beam at the receiver (i.e. reduce the size ofthe spot a the detector plane); wherein speckle-to-receiver-aperturetracking mechanisms are employed in the transmission channel of thecommunication system and laser beam speckle tracking mechanism in thereception channels thereof, so as to achieve a first level of opticalsignal intensity stabilization at signal detector of each receptionchannel; wherein speckle-to-fiber/detector locking mechanisms areemployed in signal reception channels of all terminals in thecommunication system so as to achieve a second level of optical signalintensity stabilization at signal detector of each receiving channel.

Another object of the present invention is to provide a free-spaceoptical laser communication system, wherein the receiver sends thecommunication link power signal that it is using to perform theSpeckle-to-Fiber/Detector Locking Method back to the transmitter via theoptical or other communications link, and the transmitter uses thissignal as the figure of merit in a control algorithm that drives aspatial phase or intensity modulator operating in a control loop, sothat a bright speckle in the transmitted laser beam is pointed andlocked onto the receiver aperture, while the receiver loop locks abright speckle in the received laser beam, onto the receiving fiber (ordetector).

These and other objects of the present invention will become moreapparently understood hereinafter and in the Claims to Inventionappended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS OF PRESENT INVENTION

For a more complete understanding of how to practice the Objects of thePresent Invention, the following Detailed Description of theIllustrative Embodiments can be read in conjunction with theaccompanying Drawings, briefly described below, wherein:

FIG. 1A is a schematic representation of an aircraft in communicationwith a space communication satellite by way of a free-space optical(FSO) laser communication system link, wherein various communicationplatform parameters are defined for use in modeling the atmosphericchannel and the performance of the FSO laser communication system,including the altitude of the aircraft, the velocity of the aircraft,and the altitude and orbital path of the satellite, wherein the aircraftaltitude determines the starting point of the communication link path,the aircraft velocity sets the rate at which the laser beam translatesthrough the atmosphere, and the satellite altitude and path determinethe apparent slew rate of the transmitted laser beam;

FIG. 1B is a schematic representation of a typical phase map used tocalculate aberrations in a horizontal laser communications linkpropagating through strong atmospheric turbulence;

FIG. 1C is a time sequence of laser beam intensity distributionsdetected at the receiver of a laser communications satellite, calculatedusing a simple atmosphere model of propagation of a laser beam to thesatellite at 1000 km altitude from an aircraft at 35,000 feet, whereinthe bar in each frame in the time sequence represents 10 m at thesatellite;

FIG. 1D is a typical Refractive Index Structure constant C_(n) ² profilegenerated by a simple atmosphere model, which is used to estimate theintegrated turbulence of free space along a vertical path from theaircraft at various altitudes;

FIG. 1E is a plot of versus aircraft altitude, based on the results ofone realization of turbulence estimates;

FIG. 1F1 is a plot of the atmospheric coherence length (Fried's r_(o)parameter) versus aircraft altitude, calculated using a wavelength of1550 nm;

FIG. 1F2 is a plot of isoplanatic angle versus aircraft altitude,calculated using a wavelength of 1550 nm;

FIG. 1G is a graphical representation of a phase screen generated for anaircraft altitude at an elevation of 35,000 feet, calculated using theintegrated C_(n) ² value of the link path;

FIG. 1H is the diffraction limited profile of the laser beam spot at thereceiver of a satellite (left image) compared against a set of twoexamples of the laser beam spot profile at the receiver of a satellite,wherein each image covers about an area 100 m by 100 m, and wherein thelaser beam profile in the center image shows that little power is lostdue to wavefront aberration, while the laser beam profile in the rightimage shows that significant loss occurs due to dispersion of the laserbeam;

FIG. 1I is a time series of laser beam power received at the receiver,calculated for a platform velocity of 200 m/s over about 0. Is of flighttime, showing at time 20 msec that there is one deep fade event of about10 db contained in this simulated data set;

FIG. 1J is a calculated Zernike Distribution for the rms variation ineach of the lowest 15 Zernike terms representing a computed compensationfor phase aberrations generated in the laser beam as a result of laserbeam pointing errors along the laser communication link, wherein thebulk of the aberration is shown contained in the first three Zerniketerms (i.e. two Zernike terms required for tilt compensation, and oneZernike term required for defocus compensation);

FIG. 1K is a simulation of a typical laser beam spot intensitydistribution pattern shown varying randomly at the plane of the receiveraperture as the turbulence along a 10 km communication link path changesthrough an atmosphere having C_(n) ²=1.0×10⁻¹⁴ m^(−2.3), and wherein thetransmitter and receiver apertures are both 20 cm in diameter and thewavelength is 1.5 microns, and the full speckle image is 1 m×1 m;

FIG. 1L is a simulated spot of the received laser beam at the focalplane of the receiver, having the characteristics of a speckle image,wherein the scale of the (individual bright) speckle image is 1.25μrad/pixel or 640 μrad for the full width of the image, and thediffraction limited spot size is 6 pixels, peak-to-null;

FIG. 1M is a simulated spot of the received laser beam at the focalplane of the receiver, showing a very dark portion of this speckle imagefalling onto the detector fiber core of the receiver, wherein the sizeand location of the detector or fiber core of the receiver is indicatedas a small circle, and wherein (i) the fluctuations in the receivedintensity and (ii) the signal variations due to the moving specklepattern at the focal plane, are not correlated;

FIG. 1N is a plot of log of the laser beam intensity (power) observed atthe receiver detector versus the number of reiterations taken by theLaser Beam Speckle-Tracking algorithm hereof to find a new peakintensity to follow (at the detector), graphically illustrating that thelaser beam intensity on the detector is monitored and when the intensitylevel falls below a predetermined threshold, then the line of sight ofthe laser beam is “shocked” and moved to a new location approximately atypical speckle spacing from its current location, and that this processis repeated until the detector intensity rises to an acceptable level,and thereafter the dithering of the received laser beam on the detector(i.e. fiber) is restarted, wherein one such “shocking” event is observedin FIG. 1N at about iteration number 220;

FIG. 1O is a plot of the probability density function (PDF) for thelaser beam intensity data shown in FIG. 1N, clearly showing that theLaser Beam Speckle Tracking (i.e. Following) Mechanism of the presentinvention can effectively eliminate the long tail of low power fadesobserved in the raw (i.e. uncompensated) intensity data detected at thereceiver, with occasional, intentional fades induced during there-acquisition of speckle components (i.e. energy carrying beam spotcomponents) during the Laser Beam Speckle Tracking Method of the presentinvention;

FIG. 1P is a set of calculated blur spots formed by introducing phasesteps of 0.2, 0.5 and 1.0 waves in the entrance pupil of thecommunication receiver, illustrating that when a phase step is added toa random phase screen in the entrance pupil, the result is a modulationof the speckle pattern of laser beam spot detected by the fiber ordetector;

FIG. 1Q is a series of speckle patterns observed within a detected laserbeam spot, that are sinusoidally modulated at each location by addingphase steps of 0.2, 0.4, 0.6, 0.8 and 1.0 waves to a phase screen in theentrance pupil of the receiver, illustrating that the intensity of agiven location within the speckle pattern (of the laser beam spot) canbe maximized (i.e. optimized) by adjusting the size of the phase step,in accordance with the Speckle-to-Fiber/Detector Locking Method of thepresent invention carried out at the receiver of each terminal in thecommunication system;

FIG. 1R1 shows a plot of simulated log laser beam intensity (power)observed at the receiver detector versus the number of reiterationstaken by the Speckle-to-Fiber/Detector Locking Mechanism of the presentinvention (using a 2×2 spatial phase modulation panel introduced in theentrance pupil of the receiver);

FIG. 1R2 shows a plot of simulated Log laser beam intensity (power)observed at the receiver detector versus the number of reiterationstaken by the Speckle-to-Fiber/Detector Locking Mechanism of the presentinvention (using a 4×4 spatial phase modulation panel introduced in theentrance pupil of the receiver);

FIG. 1S is a plot of the probability density function (PDF) for thelaser beam (speckle pattern) intensity data for simulations carried outusing a 4×4 spatial phase modulation panel introduced in the entrancepupil of the receiver, showing that even when using a 4×4 (low spatialresolution) spatial phase modulation panel, having only sixteen (16)degrees of freedom, the Speckle-to-Fiber/Detector Locking Mechanism iscapable of reducing significantly the occurrence of deep fades by virtueof its ability to lock the brightest laser beam speckle (in the laserbeam spot image) onto the fiber/detector of the receiver, as turbulentatmospheric conditions move past the entrance pupil of thereof;

FIG. 2A is a schematic representation of an aircraft carrying amulti-point FSO laser communication platform in accordance with theprinciples of the present invention, and supporting numerousbi-directional FSO laser communication links with multi-point FSO lasercommunication platforms carried by other aircrafts, terrestrial vehiclesand naval carriers, shown arranged in accordance with a militarycommand, control and communications (C3) environment;

FIG. 2B is a schematic representation of a city having buildings,towers, and other civil structures, on which are supported a multi-pointFSO laser communication platforms in accordance with the principles ofthe present invention, each supporting numerous bi-directional FSO lasercommunication links with others multi-point FSO laser communicationplatforms carried by civil structures arranged within a civilianenvironment;

FIG. 3A is a schematic representation of a free-space optical (FSO)laser communication system supporting optically-separated signaltransmission and reception channels, and employing Laser Beam PointingMechanisms along the signal transmission channels thereof forautomatically pointing the transmitted laser beam towards the receiveraperture of the communication system, and Laser Beam Speckle TrackingMechanisms and Speckle-to-Fiber/Detector Mechanisms along the signalreception channels thereof for automatically stabilizing variations inthe detected intensity of received laser beam carrier signals caused byatmospheric turbulence along said signal channels;

FIG. 3B is a schematic optical diagram of the free-space optical (FSO)laser communication system of FIG. 3A, showing the transmitter andreceiver module of each terminal in the system being arranged alongoptically-separated signal transmission and reception channels, andLaser Beam Pointing Mechanisms being employed along the signaltransmission channels thereof for automatically pointing the transmittedlaser beam towards the receiver aperture of the communication system,Laser Beam Speckle Tracking Mechanisms being employed in both the signalreception channels of system for automatically tracking (i.e. following)maximum intensity laser beam speckle and moving away from low intensity(i.e. black) laser beam speckle so as to achieve a first level ofoptical signal intensity stabilization at signal detector of thereceiver modules, and also Speckle-to-Fiber/Detector Mechanism in thesignal reception channels of system to automatically lock a maximumintensity laser beam speckle onto a fiber/detector, and thereby achievea second level of optical signal intensity stabilization at the signaldetector in the receiver modules of the system;

FIG. 3C is a schematic optical diagram of the transmitter module in eachterminal of the laser communication system of FIG. 3A;

FIG. 3D is a schematic optical diagram of the receiver module in eachterminal of the laser communication system of FIG. 3A;

FIG. 3E is a schematic representation of a 4×4 spatial phase modulationpanel of used in the receiver module of each terminal in the lasercommunication system of FIG. 3A;

FIG. 3F is a schematic representation of a 4 cell (i.e. quadrant-type)signal detector used in the laser beam spot tracking mechanism in thereceiver module of each terminal in the laser communication system ofFIG. 3A;

FIG. 4A is a flow chart showing the steps involved in a first method ofgenerating a control signal for supply to the fast steering mirror (FSM)used in the Laser Beam Tracking Mechanism in the transmitter module ofeach the laser communication system of the present invention;

FIG. 4B is a flow chart showing the steps involved in a second method ofgenerating a proportional control signal for supply to the FSM used inthe Laser Beam Tracking Mechanism in the transmitter module of each thelaser communication system of the present invention;

FIG. 4C is a flow chart showing the steps involved in a third method ofgenerating a proportional plus integral plus derivative control signalfor supply to the FSM used in the Laser Beam Tracking Mechanism in thetransmitter module of each the laser communication system of the presentinvention;

FIG. 4D is a flow chart showing the steps involved in a first method ofgenerating a control signal for supply to the fast steering mirror (FSM)used in the Laser Beam Speckle Tracking Mechanism in the receiver moduleof each the laser communication system of the present invention;

FIG. 4E is a flow chart showing the steps involved in a second method ofgenerating a proportional control signal for supply to the FSM used inthe Laser Beam Speckle Tracking Mechanism in the receiver module of eachthe laser communication system of the present invention;

FIG. 4F is a flow chart showing the steps involved in a third method ofgenerating a proportional plus integral plus derivative control signalfor supply to the FSM used in the Laser Beam Speckle Tracking Mechanismin the receiver module of each the laser communication system of thepresent invention;

FIG. 5A is a flow chart showing the steps involved in a firstillustrative embodiment of the method of the present invention, forgenerating spatial phase modulation panel drive signals to be providedto the spatial phase modulation panel (i.e. deformable mirror) employedin Speckle-to-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention;

FIG. 5B is a flow chart showing the steps involved in a secondillustrative embodiment of the method of the present invention, forgenerating spatial phase modulation panel drive signals to be providedto the spatial phase modulation panel (i.e. deformable mirror) employedin the Speckle-to-Fiber/Detector Locking Mechanism in the receivermodule of each laser communication system of the present invention;

FIG. 5C is a flow chart showing the steps involved in a thirdillustrative embodiment of the method of the present invention, forgenerating spatial phase modulation drive signals to the supplied to thespatial phase modulation panel (i.e. deformable mirror) employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of present invention;

FIG. 5D is a flow chart showing the steps involved in a firstillustrative embodiment of the method of the present invention, forgenerating speckle-to-fiber/detector locking drive signals to thespatial phase modulation panel (i.e. deformable mirror) employed in thereceiver module of each laser communication system of the presentinvention;

FIG. 6A is a flow chart showing the steps involved in a firstillustrative embodiment of the method of the present invention, forgenerating spatial intensity modulation drive signals to be provided tothe spatial intensity modulation panel employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention;

FIG. 6B is a flow chart showing the steps involved in a secondillustrative embodiment of the method of the present invention, forgenerating spatial intensity modulation drive signals in theSpeckle-To-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention;

FIG. 6C is a flow chart showing the steps involved in a thirdillustrative embodiment of the method of the present invention, forgenerating spatial intensity modulation drive signals to be provided tothe spatial intensity modulation panel employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention;

FIG. 6D is a flow chart showing the steps involved in a firstillustrative embodiment of the method of the present invention, forgenerating spatial intensity modulation drive signals to be provided tothe spatial intensity modulation panel employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention;

FIG. 7A is a schematic representation of a free-space optical (FSO)laser communication system supporting optically-combined signaltransmission and reception channels, and employing Laser Beam PointingMechanisms being employed along the signal transmission channels thereoffor automatically pointing the transmitted laser beam towards thereceiver aperture of the communication system, and Laser Beam SpeckleTracking Mechanism and Speckle-to-Fiber/Detector Locking Mechanismsemployed along the signal reception channels thereof for automaticallylocking a maximum intensity laser beam speckle onto a fiber/detector,and thereby stabilizing variations in the detected intensity of receivedlaser beam carrier signals caused by atmospheric turbulence along saidsignal channels;

FIG. 7B is a schematic optical diagram of the free-space optical (FSO)laser communication system of FIG. 37, showing the transceiver module ofeach terminal in the system being arranged along optically-combinedsignal transmission and reception channels, and Laser Beam PointingMechanisms being employed along the signal transmission channels thereoffor automatically pointing the transmitted laser beam towards thereceiver aperture of the communication system, Laser Beam SpeckleTracking Mechanisms being employed in both the signal reception channelsof system to achieve a first level of optical signal intensitystabilization at signal detector of the transceiver modules, and also aSpeckle-to-Fiber/Detector Mechanisms in the signal reception channels ofsystem to automatically lock a maximum intensity laser beam speckle ontoa fiber/detector, and thereby achieve a second level of optical signalintensity stabilization at the signal detector in the transceivermodules of the system;

FIG. 7C is a schematic optical diagram of the transceiver module in eachterminal of the laser communication system of FIG. 3A;

FIG. 8A is a schematic representation of a free-space adaptive optical(FS-OA) laser communication system supporting optically-separated signaltransmission and reception channels and employing Laser Beam PointingMechanisms along the signal transmission channels thereof forautomatically pointing the transmitted laser beam towards the receiveraperture of the communication system, and Laser Beam Speckle TrackingMechanisms and Speckle-to-Fiber/Detector Locking Mechanisms along thesignal reception channels thereof for automatically locking a maximumintensity laser beam speckle onto a fiber/detector, thereby stabilizingvariations in the intensity of received laser beam carrier signalscaused by atmospheric turbulence along said signal channels;

FIG. 8B is a schematic optical diagram of the FS-AO laser communicationsystem of FIG. 8A, showing the transmitter and receiver module of eachterminal in the system being arranged along optically-separated signaltransmission and reception channels having AO-compensation mechanisms,and Laser Beam Pointing Mechanisms being employed along the signaltransmission channels thereof for automatically pointing the transmittedlaser beam towards the receiver aperture of the communication system,Laser Beam Speckle Tracking Mechanisms being employed in the signalreception channels of system to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules, andalso Speckle-to-Fiber/Detector Locking Mechanism in the signal receptionchannels of system to automatically lock a maximum intensity laser beamspeckle onto a fiber/detector, and thereby achieve a second level ofoptical signal intensity stabilization at the signal detector in thereceiver modules of the system;

FIG. 8C is a schematic optical diagram of the transmitter module in eachterminal of the laser communication system of FIG. 8A;

FIG. 8D is a schematic optical diagram of the receiver module in eachterminal of the laser communication system of FIG. 8A;

FIG. 9A is a schematic representation of a free-space adaptive optical(FS-OA) laser communication system supporting optically-combined signaltransmission and reception channels and employing Laser Beam PointingMechanisms along the signal transmission channels thereof forautomatically pointing the transmitted laser beam towards the receiveraperture of the communication system, and Laser Beam Speckle TrackingMechanisms and Speckle-to-Fiber/Detector Mechanisms along the signalreception channels thereof for automatically locking a maximum intensitylaser beam speckle onto a fiber/detector of the receiver, therebystabilizing variations in the intensity of received laser beam carriersignals caused by atmospheric turbulence along said signal channels;

FIG. 9B is a schematic optical diagram of the FS-AO laser communicationsystem of FIG. 9A, showing the transceiver module of each terminal inthe system being arranged along optically-combined signal transmissionand reception channels having AO-compensation mechanisms, and Laser BeamPointing Mechanisms being employed along the signal transmissionchannels thereof for automatically pointing the transmitted laser beamtowards the receiver aperture of the communication system, Laser BeamSpeckle Tracking Mechanisms being employed in the signal receptionchannels of system to achieve a first level of optical signal intensitystabilization at signal detector of the transceiver modules, and alsoSpeckle-to-Fiber/Detector Locking Mechanism in the signal receptionchannels of system to automatically lock a maximum intensity laser beamspeckle onto the fiber/detector of the receiver, and achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the transceiver nodules of the system;

FIG. 9C is a schematic optical diagram of the transceiver module in eachterminal of the laser communication system of FIG. 9A;

FIG. 9D is a schematic diagram of the FS-OA laser communication terminalused in the laser communication system of FIG. 9A;

FIG. 9E is a photograph of a tripod-mounted FS-OA laser communicationterminal constructed in accordance with the architecture of FIG. 9B,wherein the transceiver telescope has a 20 cm aperture and is mountedbeneath the optical bench of the terminal, and a high-spatial resolutionAO (WFS/WFC) compensation subsystem is provided for communication linkcharacterization/compensation, and a real-time subsystem is provided forstabilizing the intensity of laser beam carrier signals detected at thetransceiver of the system;

FIG. 9F is a photograph of a compact FS-OA laser communication terminalalso constructed in accordance with the architecture of FIG. 9B, whereinthe transceiver telescope has a 15 cm aperture and is mounted beneaththe optical bench of the terminal, and a high-spatial resolution AO(WFS/WFC) compensation subsystem is provided for communication linkcharacterization/compensation, and a real-time subsystem having a LaserBeam Speckle Tracking Mechanism (including a fast steering mirror FSM))and a Speckle-to-Fiber/Detector Locking Mechanism (including adeformable mirror as a spatial phase modulation panel) are provided forautomatically locking a maximum intensity laser beam speckle onto afiber/detector of the receive, thereby stabilizing the intensity oflaser beam carrier signals detected at the transceiver of the system, inresponse to atmospheric turbulence;

FIG. 10A is a schematic representation of a free-space adaptive optical(FS-OA) laser communication system supporting optically-combined signaltransmission and reception channels and employing Laser Beam PointingMechanisms along the signal transmission channels thereof forautomatically pointing the centroid of the transmitted laser beamtowards the receiver aperture of the communication system,Speckle-To-Receiver-Aperture Locking Mechanisms along the transmissionchannels for locking laser beam speckle in the transmitted laser beamonto the receiver aperture of the receiver module in the system, andLaser Beam Speckle Tracking Mechanisms and Speckle-to-Fiber/DetectorLocking Mechanisms along the signal reception channels thereof forlocking a maximum intensity laser beam speckle onto the fiber ordetector of the receiver, thereby automatically stabilizing variationsin the intensity of received laser beam carrier signals caused byatmospheric turbulence along said signal channels; and

FIG. 10B is a schematic optical diagram of the FS-AO laser communicationsystem of FIG. 10A, showing the transceiver module of each terminal inthe system being arranged along optically-separated signal transmissionand reception channels having AO-compensation mechanisms, and Laser BeamPointing Mechanisms being employed along the signal transmissionchannels thereof for automatically pointing the centroid of thetransmitted laser beam towards the receiver aperture of thecommunication system, Speckle-to-Receiver-Aperture Locking Mechanismsbeing employed in the signal transmission channels of the system, andLaser Beam Speckle Tracking Mechanisms being employed in the signalreception channels of system to achieve a first level of optical signalintensity stabilization at signal detector of the transceiver modules,and also Speckle-To-Fiber/Detector Locking Mechanisms in the signalreception channels of system to lock a maximum intensity laser beamspeckle onto the fiber or detector of the receiver, and thereby achievea second level of optical signal intensity stabilization at the signaldetector in the transceiver modules of the system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENTINVENTION

Referring to the figures in the accompanying Drawings, the variousillustrative embodiments of the hand-supportable imaging-based bar codesymbol reading system of the present invention will be described ingreat detail, wherein like elements will be indicated using likereference numerals.

General Description of Laser Communications Link Fade PreventionTechniques of the Present Invention

Through research, Applicants have discovered that the spatial intensityvariations in the received laser beam signal at the input plane of thereceiver lead to temporal fluctuations in the total power received. FIG.1K shows a simulation of a typical intensity distribution at the planeof the receiver aperture. This case is for a 10 km path through anatmosphere with C_(n) ²=1.0×10⁻¹⁴ m^(−2.3). The transmitter and receiverapertures are both 20 cm in diameter and the wavelength is 1.5 microns.The full image shown is 1 m×1 m. This intensity pattern varies randomlyas the turbulence along the path changes.

Clearly, unless the receiver is made very large, there will besignificant variations in the received power. The receiver acts to focusthe received laser beam spot onto a detector, either directly or via afiber optic. In either case, the focal spot (of the laser beam) must bevery small to match the size of the detector or fiber core. Because ofthe phase aberrations in the input signal to the receiver, the focalspot is aberrated and will always be larger than the diffraction limit.

FIG. 1L shows the simulated spot of the received laser beam at the focalplane of the receiver. This simulated spot has the characteristics of aspeckle image. The scale here is 1.25 μrad/pixel or 640 μrad for thefull width of the image. The diffraction limited spot size is 6 pixels,peak-to-null. For details on the nature of speckle images produced bycoherent laser beams, reference is made to WIPO Publication No. WO02/43195 A2 by Applicants, which is incorporated herein by reference inits entirety.

Occasionally, a very dark portion of this speckle image will fall ontothe fiber or detector of the receiver in the communication terminal.Such a case is shown in FIG. 1M. The small circle in FIG. 1M indicatesthe size and location of the detector or fiber core of the receiver. Thefluctuations in the received intensity and the signal variations due tothe moving speckle pattern at the focal plane are not correlated. Thusit will sometimes happen that at a time when the total received power islow a dark part of the speckle pattern will be over the detector. Duringsuch events the power reaching the detector may be reduced by a factorof more than 10³ relative to the average power. It is very difficult todesign a communications link that can maintain a low bit error rate(BER) during these deep fade events.

A primary object of the present invention is to provide severaltechniques for the reducing signal fading in optical free spacecommunication links. Fading of the signal due to the effects ofatmospheric turbulence is a well_documented problem for the opticalcommunications links, and has been studied in great detail at AdaptiveOptics Associates, Inc., and described hereinabove.

In general, atmospheric turbulence induces wavefront aberrations thatevolve so that both the phase and intensity distributions across thelaser beam become non-uniform. This can lead to two problems for thecommunications link, namely: (1) the spatial intensity variations of thelaser beam generated at the input plane of the receiver by channelturbulence lead to temporal fluctuations in the total power received atthe detector; and (2) this spatial intensity pattern of the receivedlaser beam varies randomly as turbulence along the laser beam pathchanges, producing a moving speckle image at the focal plane of thereceiver, having a very dark portion which may fall upon the receiver'sfiber or detector, causing the total received laser beam power to dropto extremely low levels when the dark part of the speckle pattern ispassing over the detector. These problems will be described in greaterdetail below.

The new methods disclosed here, while somewhat related to traditional AOsystems, are based on a reassessment of the basic requirements of thefree-space optical (FSO) laser communications system. AO basedcompensation systems are designed to produce a near diffraction limitedspot after passage through a turbulent atmosphere. That design is drivenby the requirement either for high resolution imaging (astronomical AO),or for maximum power density in the spot (directed energy weapons). Ifthat could be achieved for a long horizontal path, it would satisfy therequirements for a stable communications link. However, a communicationslink is neither an imaging system nor a weapon system. The requirementis not to minimize the spot size or maximize the power density butrather to minimize the variations in the power. In a typicalcommunications link the average received power is more than adequate toachieve a very low BER. The power level is driven in part by the fadingproblem. To assure that the link will function during a fade to 5% ofthe average power, the average power level is set 20× the minimum neededfor the required BER.

The methods described here are aimed at stabilizing the power on thedetector rather than maximizing that power. The first uses adjustment ofthe line of sight to try to keep a bright portion of the speckle patternon the fiber or detector. The second uses phase modulation in the pupilplane to modify the speckle pattern in ways that help to stabilize thereceived power.

Laser Beam Speckle Tracking (i.e. Following) Method and Mechanism of thePresent Invention

The characteristics of the speckle pattern at the detector plane aredetermined by the atmospheric aberrations and the optical system thatproduced the pattern. The overall width of the pattern is related to thestrength of the aberrations. The typical size of the individual specklesis similar to the size of the diffraction limited spot for particularoptical system. Suppose that a speckle pattern such as the one shown inFIG. 1M falls on a fiber or detector. Assume that the pattern issignificantly larger than the fiber or detector. If the laser beamspeckle pattern is moved to three or more positions separated bydistances less than a typical speckle size and the received powerrecorded at each location, then it is possible to calculate the localgradient of the intensity pattern. The laser beam speckle pattern isthen moved in the direction of the positive gradient by a distance lessthan a speckle size and the estimation of the gradient repeated. If thespeckle pattern did not change, then the result of repeating thisprocess several times would be to move the speckle pattern so that apeak of a laser beam speckle was centered on the fiber or detector. Inthe realistic case of an evolving speckle pattern, as long as thegradient estimation and pattern repositioning is performed several timeswithin the timescale of speckle evolution, the result will be to keepthe fiber or detector close to the peak of the laser beam speckle.

This is essentially a hill climbing algorithm that uses dithering of theposition of the laser beam speckle pattern to determine the directiontowards the local maximum of intensity. Simulations of this algorithmwere performed to assess its performance. A single phase screen with aKolmogorov spectrum was used. A large screen was generated andsubsections extracted along a linear path to simulate turbulence movingpast the input aperture. Both the strength of the phase screen and therate at which the screen is moved are input parameters to thesimulation. For each extracted phase screen, the point spread function(PSF) was calculated. Dithering of the image was simulated by looking atthe intensity at four nearby locations in the PSF. A plane was thenfitted to the intensities at those points. This fit was then used tocalculate the local gradient of the intensity. The effective line ofsight of the system was then updated based on the gradient and a newphase screen extracted. This process was repeated typically for 1000iterations. For each iteration, the power falling on the “fiber” or“detector” was recorded. At the same time, the power falling on anuncompensated detector using the same phase screens was calculated forcomparison with the compensated power. FIG. 9G shows a typical resultplotting the log of the power versus iteration number for thecompensated (green) and uncompensated (red) data. This simulation used a10 cm receiver at 1.5μ wavelength and a phase screen with Friedparameter (r₀) of 1 cm. The phase screen is moved by 1 mm betweeniterations, which is roughly equivalent, at a wind speed of 10 mph, to atime step of 225 μs.

These simulations uncovered a shortcoming of the Laser Beam SpeckleTracking (i.e. Following) Method of the present invention. The hillclimbing algorithm was effective in keeping the peak of a laser beamspeckle centered on the fiber or detector for considerable periods oftime. However, eventually a situation arises in which the laser beamspeckle simply fades away without moving. The hill climbing algorithm isthen cast adrift and must work its way up to the peak if a new laserbeam speckles. If, in the course of following a speckle, the fiber ordetector location had moved far from the average center of the PSF, thenit may take a large number of iterations for the algorithm to find a newpeak to follow.

A crude solution to this problem was implemented in the simulation usedto generate FIG. 9G. The intensity on the detector was monitored and ifit fell below some threshold, then the line of sight of the system was“shocked” and moved to a new location approximately a typical specklespacing from its current location. This was repeated until the intensityrose to an acceptable level and then the dithering restarted. One suchevent is seen in FIG. 9G at about iteration 220. One further enhancementwas added to the Method. If the distance of the detector from theaverage center of the PSF was above some value, then the fiber ordetector was re-centered and the algorithm restarted. It should be notedthat these “shock” events can lead to fades in the signal until a laserbeam speckle is re-acquired. Fortunately, they are anticipated eventsand the communications link could be interrupted in an orderly fashion.

FIG. 9H shows the probability density function (PDF) for the data inFIG. 9G. This clearly shows that the Laser Beam Speckle Tracking Methodcan effectively eliminate the long tail of low power fades seen in theraw data. The price paid for this improvement aft is the occasional,intentional fades during speckle reacquisition.

The Laser Beam Speckle Tracking Method of the present invention ispracticed in all the FSO laser communication systems shown in FIGS. 3Athrough 10B.

Speckle-to-Fiber/Detector Locking Method of the Present Invention

While the Laser Beam Speckle Tracking (i.e. Following) Method of thepresent invention showed clear improvement in the stability of thereceived power, it was not entirely free of fading problems. Analternative approach was then examined using the same simulation tools,as described below.

It is well known that application of a phase step in the entrance pupilof an imaging system alters the diffraction limited image in a veryparticular way. FIG. 9L shows the PSFs resulting from the application ofphase steps 0.2, 0.5 and 1.0 waves in the entrance pupil. If a phasestep is added to a random phase screen in the pupil, the result ismodulation of the speckle pattern. FIG. 9I shows the result of addingphase steps of 0.2, 0.4, 0.6, 0.8 and 1.0 waves to a phase screen thatproduces a speckle pattern. The result is that the intensity ismodulated sinusoidally at each location. This phenomenon leads to theconcept of the second approach to intensity stabilization. As shown inFIG. 9J, the size of the phase step can be adjusted to maximize theintensity at a given location within a speckle pattern. As the specklepattern evolves, the optimum phase step can be discovered using the samedithering and hill climbing technique used in the speckletracking/following method described above. Here, however, it is the sizeof the phase step that is dithered while monitoring the intensity on thedetector. This very simple arrangement was simulated and was found tohave the same problem that the speckle following technique exhibited.While it can maximize the brightness of a speckle on the detector, ifthat speckle fades away, then even its maximum brightness falls belowacceptable levels.

The single variable phase step may be thought of as an adaptive optics(AO) system of sorts, having a single degree of freedom, but without theuse of wavefront sensing. It is known that an AO system with a number ofdegrees of freedom larger than the number of coherence cells in thepupil can, in principle, compensate the image so that it is close todiffraction limit. A question that can be asked is: Is there a systemwith a number of degrees of freedom between these limits that canstabilize the intensity of the laser beam sufficiently to be useful in alaser communications system?

To answer that question, a simulation was constructed that used phasescreens with 4, 9 and 16 degrees of freedom arranged as square arrays of2×2, 3×3, and 4×4 independent piston levels in the pupil of the imagingsystem, respectively. The piston values of each element were ditheredwhile the laser beam speckle intensity on the fiber or detector wasmonitored. Each element was then displaced in the direction that led tothe highest power on the fiber or detector. The results of thesesimulations show that even with as few as four degrees of freedom it ispossible to very significantly reduce the occurrence of deep fades.FIGS. 9K1 and 9K2 shows the log intensity plots for simulations of 2×2and 4×4 phase modulators, respectively. The simulation parameters arethe same as those for FIG. 9G. Thus, this Speckle-to-Fiber/DetectorLocking Method described above appears to have great promise as anapproach to the elimination of deep fades in a FSO laser communicationslink. Based on preliminary analysis of system performance, a 4×4 spatialphase modulator can keep the received power stabilized within a 10 dbband under conditions that produce fades deeper that 30 db for anuncompensated system.

The Speckle-to-Fiber/Detector Locking Method described above iscomparatively easy to implement using either spatial phase or intensitymodulation at the entrance pupil of the receiver. The key element is thespatial phase or intensity modulator itself. There are several wellknown methods for introducing spatial phase or intensity modulation on alaser beam at the entrance pupil of the receiver. The most common aredeformable mirrors (DMs) for spatial phase modulators, and liquidcrystal modulators for spatial intensity modulators. Within the DMcategory, there are traditional piezoelectric driven mirrors and themore recently developed micro-machined (MEMS) versions. To determinewhich is best suited to this technique, a set of baseline requirementshas been developed.

Employing Spatial Phase Modulation at the Entrance Pupil of the Receiverto Implement the Speckle-to-Fiber/Detector Locking Method of the PresentInvention

Measurements of the temporal power spectrum of turbulence alonghorizontal paths often show significant power at frequencies above 1kHz. To compensate for such turbulence, the correction system of thepresent invention (i.e. Speckle-to-Fiber/Detector Locking Mechanism)must have a response time at least ten times faster than thefluctuations in the atmosphere, or at most 100 μs. During one responsetime interval, it is necessary to perform all the dithering of thevarious degrees of freedom and the associated data processing.

For the case of the 4×4 spatial phase modulator this means that 16cycles of the dither motion must occur in that interval. This implies aminimum frequency response of the spatial phase modulator of 160 kHz.Fortunately, the spatial phase modulator can be made quite small bypositioning it at a relayed pupil image plane. Since the system ismonochromatic, the phase modulator only needs a dynamic range of +−½wave (the simulations described here limit the spatial phase modulatorto this range).

There are currently available piezo-type actuators in sizes of severalmillimeters on a side that have resonant frequencies of 300 kHz andgreater than 2 μm throw. By bonding thin, flat mirrors onto an array ofthese actuators, a suitable phase modulator can be fabricated. FIG. 3Eshows a schematic view of a spatial phase modulator constructed usingsuch components. The bonding of the mirrors may be done with the mirrorsin contact with an optical flat to assure alignment.

Alternatively, there are developmental MEMS spatial phase modulatorsthat approach 100 kHz resonant frequencies with a cell size of 0.3 mm.If such modulators prove practical, then they offer advantages in powerconsumption and cost.

Intensity data is required at rates similar to the dither frequency ofthe spatial phase modulator, but can easily be supplied by thecommunications link detector itself since it must be capable ofoperation at frequencies in the GHz range.

FIGS. 3D and 3E show an illustrative embodiment for implementing theSpeckle-To-Fiber/Detector Locking Method of the present invention. Asshown in FIG. 3D, it is optically similar to a raditional AOcompensation system using a deformable mirror (DM) in a relayed pupilplane. It also includes a fast steering mirror (FSM) to remove the fullaperture tilt component of the aberrations. This FSM control loop doesnot have to operate at the very high frequencies of the spatial phasemodulator (DM), but it does need a control bandwidth approaching 1 kHzfor operation under conditions of strong turbulence.

The data processing requirements for implementing theSpeckle-to-Fiber/Detector Locking Method are modest. The intensity datasteam does not need to be faster than about 1 MHz. During each dithercycle of the spatial phase modulator, the processor must test to seewhat direction of modulator motion produces a higher signal and apply anoffset in that direction to the appropriate actuator. It also mustassure that the offset signals to the phase modulator are kept withinthe ±½ wave region. This can be achieved by either a look-up table or amathematical function that calculates the drive signal equivalent to onewave. Since modern processors can perform more than 1000 operations inthe time for a single signal update, this is not seen as a problem.

Employing Spatial Intensity Modulation at the Entrance Pupil of theReceiver to Implement the Speckle-to-Fiber/Detector Locking Method ofthe Present Invention

Another way of improving the information content of the laser beamspeckle image received at the entrance pupil of an FSO lasercommunication receiver involves active masking of the entrance pupil ofthe receiver using spatial intensity modulation techniques.

A typical atmospherically induced wavefront has about 5 waves ofaberration, peak-to-valley. During simulations, this wavefront can begenerated in such a way that it has a Kolmogorov distribution.Examination of this wavefront reveals that there are regions within thewavefront that are relatively flat. These regions may, however, betilted fairly significantly. If that region is extracted from thewavefront and its tilt removed, the residual wavefront error is found tobe less than _wave. If the aperture of the imaging system were maskeddown to allow only this portion of the wavefront to enter the receiver'spupil aperture, then a much improved image would result.

It has been discovered that the masking of the pupil results in a markedimprovement in laser beam image quality. However, it must be realizedthat the location and size of such masks depends on the instantaneousatmospheric wavefront in the pupil. For this technique to be practical,there must be a way to actively select the region to be allowed to formthe image. One approach is to look for flat parts of the wavefront.Unfortunately, this would require knowledge of the shape of thewavefront, and there are difficulties associated with using wavefrontsensing for this purpose. An alternative method is to use a mask that isdithered while the laser beam image quality is monitored at the entrancepupil. In accordance with the present invention, the entrance pupil canbe broken up into many smaller regions and each of these alternatelymasked and unmasked. The laser beam spot image quality for each state ofthe mask is recorded and the mask for that region is set to the statethat results in the higher image information content. This process isrepeated for each of the sub-regions until the whole entrance pupil iscovered. This process can be carried out using essentially a hillclimbing algorithm with the goal of maximizing the laser beam spot imageinformation.

To assess the performance of such a hill climbing algorithm, a computersimulation was constructed. The simulation models a 10 cm receiveraperture operating at a wavelength of 0.9 μm. Phase aberrations areintroduced into the pupil of the receiver. These aberrations aregenerated by extracting parts of a large phase screen that hasKolmogorov distributed aberrations. Four separate regions are extractedand added to produce the pupil wavefront. The extraction points aremoved across the large phase screen to simulate the effect of wind. Eachof the four extraction regions moves in a different direction. Both thestrength of the phase screen and the effective wind speed can be varied.The simulated pupil wavefront is then used to calculate the laser beamblur spot formed by the receiver telescope. This blur spot is thenconvolved with a high resolution image of the scene to simulate theeffect of the aberration on laser beam spot image quality.

In this simulation, image contrast is used as a measure of laser beamimage quality. A small (64×64 pixels) region of the laser beam image isextracted and the spatial gradient of the intensity is calculated. Themetric of laser beam image quality is the ratio of the rms of themagnitude of the gradient divided by the average image intensity.

The intensity function in the receiver pupil is assumed uniform. Thespatial light modulator (SLM) is simulated as an N×N array of squaresubapertures whose transmission can be set to 1.0 (open) or to someother value (closed). The closed value can be 0.0 to simulate a highcontrast modulator or to larger values to simulate lower contrastmodulators such as quantum-well devices.

The simulation begins with all subapertures in the open state. One at atime, each subaperture is closed and the change in image quality metricis recorded. If the metric drops, the subaperture is opened again, ifthe metric increases, then the subaperture is kept closed. After all thesubapertures have been tested in this way, a new pupil wavefront isextracted from the large phase screen. The process is repeated. Howeverfor subsequent iterations, the state of each subperture is inverted fromits current state and the effect on the metric is noted. Those changesthat lead to increased metric are retained, all other subapertures arereturned to their initial state.

Simulations have been run using modulators with resolutions from 5×5 to20×20 with phase screens simulating turbulence with C_(n) ² in the rangeof 10⁻¹⁵ to 10⁻¹⁴ m^(−2/3). The simulation used a 7×7 element SLM and anatmospheric path with r_(o)=1.5 cm. The effective wind speed was suchthat the phase screens moved about 2.5 mm per iteration. For a windspeed of 10 mph, this is equivalent to an iteration rate of 2 kHz. Tosimulate the laser beam spot image seen by a video rate sensor, thelaser beam image data was averaged into bins of about 16 ms (60 Hz framerate). The images are a time sequence of raw (bottom) and corrected(top) simulated video output. The image contrast metric for all 1000iterations of this simulation is measured. It has been found that theimage contrast metric is improved by an average factor of 2.44 with peakimprovement of a factor of 4.6. If the image contrast is taken to beinversely proportional to the smallest image feature resolved, then anincrease in contrast by a factor of 2.44 represents an increase in laserbeam image information by a factor of 6.

Alternative pupil masking techniques and image quality metrics may beused. One approach that has been tested in the simulation is to use apupil mask of fixed size whose position is dithered. Again, a hillclimbing algorithm is used to determine the mask position that resultsin the best image quality. The performance of this technique appears tobe similar to that of the subaperture mask. Several different imagequality metrics have been tested in the simulation. These include thenormalized rms amplitude of the gradient of the intensity, the maximumintensity gradient, and intensity gradient histogram based metrics.

The Speckle-to-Fiber/Detector Locking Method described above usingspatial intensity modulation techniques at the entrance pupil of thereceiver is comparatively easy to implement. The key element is themodulator itself. There are several well known methods for introducingintensity modulation on a beam. The most common are liquid crystalmodulators and deformable mirrors (DMs). More recently, quantum wellmodulators have been developed that offer higher speed operation butwith lower on-off contrast ratio. To determine which is best suited tothis technique, a set of baseline requirements has been developed.

Measurements of the temporal power spectrum of turbulence alonghorizontal paths often show significant power out to frequencies well of100 Hz. To compensate for such turbulence the correction system musthave a response time at least ten times faster than the fluctuations inthe atmosphere, or at most 1000 μs. During one response time interval itis necessary to perform all the dithering of the various degrees offreedom and the associated data processing. For the case of the 5×5intensity modulator this means that 25 cycles of the dither motion mustoccur in that interval. This implies a minimum frequency response of thephase modulator of 25 kHz. Fortunately, the modulator can be made quitesmall by positioning it at a relayed pupil image plane.

There are currently available DM-based spatial light modulators that arecapable of switching at the rates required by thisSpeckle-to-Fiber/Detector Locking Technique. One example is the TexasInstruments DLP chip. This device uses micro-machining techniques toproduce an array of small tilting mirrors. These mirrors may be flippedbetween the on state, in which they reflect the light into the remainderof the optical system, and the off state, in which the light is directedto an optical stop.

It has been found that, at this time, conventional liquid crystalmodulators are too slow to be applied to this technique, being limitedto speeds below 1 kHz. Quantum well modulators are capable of speedswell above 1 MHz, but provide contrast ratios of only 2:1 or 3:1 betweenstates. However, initial simulation results indicate that theperformance of this technique is only moderately degraded by using a 3:1contrast ratio modulator.

The data processing requirements for using spatial intensity modulationtechniques at the entrance pupil of the receiver to realize theSpeckle-to-Fiber/Detector Locking Method of the present invention, areconsiderable, but within the capabilities of present PC processors. A40×40 pixel high speed image sensor would have a data rate of 40 MHz.For each frame of data a image quality metric must be calculated. A 1GHz processor can perform 25 operations on each pixel of the data. Thealgorithm used in the simulations performs about 5 operations per pixel.During each dither cycle of the modulator, the processor must test tosee what state of the modulator produces a higher image metric and, onthe basis of the signal change, set the state of the appropriateactuator. This part of the algorithm needs only to operate at the 25 kHzupdate rate and is not a problem.

The Speckle-to-Fiber/Detector Locking of the present invention ispracticed in all FSO laser communication systems shown in FIGS. 3Athrough 10B.

Speckle-to-Receiver-Aperture Locking Method of the Present Invention

Over long paths with significant turbulence, the laser beam spot patternfrom a laser communication terminal may evolve into a speckle pattern.These spatial intensity variations at the input plane of the receiverlead to temporal fluctuations of the total power received. Applicantshave constructed simulations of a typical intensity distribution at theplane of the receiver aperture (e.g. for a 10 km path through anatmosphere with C_(n) ²=1.0×10⁻¹⁴ m^(2/3)). In the simulation, thetransmitter and receiver apertures are both 20 cm in diameter and thewavelength is 1.5 microns. The full image shown is 1 m×1 m.

The size of the individual speckles in the laser beam spot is about thediffraction limit of the full transmitter aperture, and the overallpattern size is set by the diffraction limit of the atmosphericcoherence length. In general, an atmospheric compensation system with asingle plane of correction (single DM) cannot correct both the phase andintensity in a beam that has passed through a long path with distributedturbulence. Therefore, even with compensation, it has been discoveredthat there will be intensity variations in the receiver input plane.

Under some circumstances, the laser beam speckle size will exceed thereceiver aperture size. When this occurs, there is the possibility forsignificant signal fade events as a dark speckle covers the aperture.These deep fades adversely affect the communications link performance,because if no light enters the aperture, then no light will reach thefiber or detector at the receiver.

In situations where the laser beam has become a speckle pattern at thereceiver, it is also true that the receiver is in the far-field of thetransmitter aperture. This means that the transmitter appears as a pointsource to the receiver and that phase aberrations introduced close tothe transmitter (including those of the transmitter AO system) havediffracted into a spherical wavefront with spatial intensity variations.Aberrations introduced onto the beam close to the receiver will still beobserved as phase variations across the laser beam, but since they havenot evolved significantly they do not produce large intensityvariations.

While the transmitter cannot affect the wavefront at the receiver, itcan alter the speckle pattern at the receiver. That pattern is theresult of the propagation of the wavefront from the transmitter apertureto the receiver. The phase of that wavefront is the addition of theatmospherically induced phase variations near the transmitter apertureand the wavefront generated by the transmitter. As has been demonstratedfor the case of fade prevention at the receiver, it is possible to alterthe speckle pattern considerably by changing the wavefront phase by lessthan a fraction of the wave. Thus another aspect of the presentinvention involves the communication of a figure of merit (typically thereceived power) from the receiver to the transmitter so that thetransmitter can dynamically point and lock a maximum intensity specklein the transmitted laser beam onto the receiver aperture of receiver onthe other side of the laser communication link, to optimize thetransmission of laser beam power into the entrance pupil of thecommunication receiver. This method can be carried out by dithering thetransmitted wavefront and monitoring a figure of merit used in thereceiver to achieve fade prevention. Preferably, the figure of meritused by the algorithm is the received power at the other end of thelaser communication link. This data may be sent to the transmitter viathe optical communications link or by other communications means.

The situation is slightly more complicated if the transmitter is alsooperating as a receiver with its own fade prevention and/or AO system.If it is using just a fade prevention system, the algorithm will need touse both its local figure of merit and that sent from the receiver.Since there will typically be many degrees of freedom available in thespatial phase or intensity modulator, it will, in general, be possibleto optimize the received power at both ends of the laser communicationlink.

If the transmitter is operating an adaptive optical correction system aspart of its receiver, then the control algorithm will have to compromisebetween optimal local received power and fade elimination at the distantreceiver. The phase modulation is required to effectively prevent fadesat the far receiver as typically small enough that they will not reducethe local received power by a large factor. The algorithm can betailored to achieve the lowest two-way BER for the full duplex link.

One issue to be considered is the latency associated with thetransmission of the figure of merit data across the link. For a linklength of 300 km the time from when the light leaves the transmitter andacquires its aberrations and when it reached the receiver is 1 ms. Itwill take another 1 ms for the data to return. To this must be added thelatency of the figure of merit detection and calculation. Thisaccumulated latency will limit the bandwidth of the fade preventionsystem.

This Speckle-to-Receiver-Aperture Locking Method of the presentinvention is practiced in the FSO laser communication system shown inFIGS. 10A and 10B.

Brief Summary of Speckle-Control Based Methods of Present Invention forReducing Fading in FSO Laser Communication Systems

In summary, three generalized methods and apparatus have been describedabove for reducing fading of laser beam signal intensity in FSO lasercommunication systems, namely: (1) automatically tracking laser beamspeckle at the receiver of any terminal in a FSO laser communicationsystem so to avoid dark speckle spots from dwelling on the fiber ordetector thereof and causing deep fades in the communication link; (2)automatically locking maximum intensity laser beam speckles onto thefiber or detector thereof so as to stabilize fluctuations in theintensity of a laser bean carrier detected at the receiver of thecommunication terminal of the present invention; and (3) automaticallylocking a maximum intensity laser beam speckle onto the receiveraperture of the laser communication system by measuring a figure ofmerit (e.g. signal intensity strength) at the receiver and communicatingthis information to the laser beam transmitter, so the transmitter canpoint a maximum intensity laser beam speckle onto the receiver apertureof the laser communication system and lock the same in a closed loopsystem.

It is appropriate at this juncture to now describe five generalizedembodiments of FSO laser communication systems shown in figures in whichsuch inventions can be embodied to provide remarkable levels of fademitigation. Notably, each of these generalized system embodiments can bepracticed in either military or civilian kinds of communicationenvironments shown in FIGS. 2A and 2B, respectively.

Free-Space Optical (FSO) Laser Communication System SupportingOptically-Separated Signal Transmission and Reception Channels, andEmploying Laser Beam Speckle Tracking Mechanisms andSpeckle-To-Fiber/Detector Mechanisms Along the Signal Reception Channelsthereof for Automatically Stabilizing Variations in the DetectedIntensity of Received Laser Beam Carrier Signals Caused by AtmosphericTurbulence Along said Signal Channels

In FIG. 3A, there is shown a first illustrative embodiment of afree-space optical (FSO) laser communication system 1 in accordance withthe principles of the present invention, wherein first and secondcommunication terminals 2A and 2B are in communication by way of a superbroad-band FSO laser beam communication link 3 havingoptically-separated signal transmission and reception channels, andwherein Laser Beam Speckle Tracking Mechanisms andSpeckle-to-Fiber/Detector Mechanisms are employed along the signalreception channels of each communication terminal, for automaticallystabilizing variations in the detected intensity of received laser beamcarrier signals caused by atmospheric turbulence along said signalchannels. As shown in FIG. 3A, each communication terminal has atransmitter (module) 4 and a receiver (module) 5, each of which will bedescribed in detail below.

The transmitter in each terminal includes a telescopic transmittingaperture, and an optical train embedded with the components of a LaserBeam Pointing Mechanism realized by a fast steering mirror and aquad-cell detector of the kind shown in FIG. 3F, supporting optics, anda processor for carrying out one of the tracking algorithms shown inFIGS. 4A through 4C.

The receiver in each terminal includes a telescopic receiving aperture,and an optical train embedded with the components of two fade mitigationmechanisms of the present invention, namely: the Laser Beam SpeckleTracking Mechanism hereof realized using a FSM, a quad-cell detectorshown in FIG. 3F, supporting optics, and a processor carrying out thetracking algorithms shown in FIGS. 4D through 4F; and theSpeckle-To-Fiber/Detector Locking Mechanism hereof realized usingspatial phase or intensity phase modulator shown in FIG. 3E, a receivingfiber, a single cell detector, and the processor carrying out (i) one ofthe spatial phase modulation (SPM) control signal generation algorithmsshown in FIGS. 5A through 5D when using spatial phase modulation (SPM)techniques, or (ii) one of the spatial intensity modulation (SIM)control signal generation algorithms shown in FIGS. 6A through 6D whenusing spatial phase modulation (SPM) techniques.

The object of the Laser Beam Speckle Tracking Mechanism employed in boththe signal reception channels of system is to automatically track orfollow a maximum intensity laser beam speckle and move away from lowintensity (i.e. black) laser beam speckles (that might fall onto thereceiving fiber) so as to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules. Theobject of the Speckle-to-Fiber/Detector Mechanism in the signalreception channels of system is to lock a maximum intensity speckle inthe received laser beam onto the receiving fiber, so as achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the receiver modules of the system.

Notably, the purpose of the tracker/FSM control loop is different in thereceiver and transmitter.

In the receiver, the tracker/FSM control loop is used to realized theLaser Beam Speckle Tracking Mechanism of the present invention which,when using the control algorithm of FIG. 4D, produces a signal that isproportional to the displacement of the received focal spot from thelocation of the receiver fiber or detector. This signal is typicallygenerated by forming an image with a portion of the received light on aquad-cell or other position sensitive imaging device. The tracker(quad-cell detector) is calibrated so that its null position isoptically identical to the position of the receiver fiber. Thedisplacement signal is the input to a control loop algorithm that drivesthe steering mirror to keep the received focal spot on the fiber. Any ofthe standard control algorithms shown in FIGS. 4D through 4F may beused.

In the transmitter, the tracker/FSM control loop is used to realize theLaser Beam Pointing Mechanism. Here, the goal of the transmittertracking control loop is to keep the transmitted beam on the receiveraperture. This is accomplished by having the tracker (quad celldetector) measure the displacement between the bore-sight direction ofthe transmitter and some optical signal received from the receiver. Anyof the standard control algorithms shown in FIGS. 4A through 4C may beused. This optical signal can be a separate beacon, as shown in FIG. 3C,or it may be a portion of the light from the transmitter associated withthe receiver. The hardware implementation of either tracker subsystem isthe same, the only differences are the source of the light used by thetracker and the definition of the null position of the tracker.

Methods of Generating a Control Signal for Supply to the Fast SteeringMirror (FSM) Used in the Laser Beam Tracking/Pointing Mechanism in theTransmitter Module of Each the Laser Communication System of the PresentInvention

In this simple form of a laser beam pointing/tracking system, a lens isused to form an image of the target (the beacon source or a portion ofthe communication link light) on a detector divided into four quadrants,as shown in FIG. 3F. Each quadrant produces a signal proportional to theintensity of light reaching the quadrant. The four resulting signals canbe combined to produce two composite signals that are estimates of theposition of the focal spot relative to the junction between thequadrants:Horizontal Position Estimator=(S1−S2)+(S4−S3)/(S1+S2+S3+S4)+K _(H)Vertical Position Estimator=(S1−S4)+(S2−S3)/(S1+S2+S3+S4)+K_(V)

Where K_(H) and K_(V) are positional offsets used to remove optical andmechanical alignment errors between the quad cell and the desired spotlocation.

If the light entering the tracking system has passed through a tiltcontrol device, such as a steering mirror driven by piezoelectricactuators, these position estimators can be used as the error inputsignals to a control algorithm. Typical control system algorithms applydrive signals to the steering mirror that are proportional to acombination of the current error signal and the integral of previouserror signals. The tilting action of the mirror provides opticalfeedback to the control system by altering the position of the image onthe quadrant detector. Thus the system will tend to drive the focal spotto a location that equalizes the four signals, S1, S2, S3, S4, makingthe position estimator values close to zero. An offset may be applied tothe signals to cause the control loop to drive the spot to otherlocations. This allows the system to compensate for alignmentdifferences between the tracker optical axis and the axis of the otherportion of the optical system.

More complex arrangements are possible. A larger array of detectorpixels may be used and their signals combined to form an estimate of thecentroid of the intensity pattern of the focal spot. Alternatively, thesignals may be used in a least-squares fitting algorithm or matchedfilter algorithm to provide estimates of the spot position.

If the focal spot has significant intensity structure (e.g. is aresolved image of the target) the signals from the detector may be usedin a correlation algorithm. This algorithm performs a correlation of theinstantaneous intensity distribution with an average or predefinedtarget intensity distribution to form the estimate of target imagelocation.

FIG. 4A describes the steps involved in a first method of generating a“proportional control” signal for supply to the fast steering mirror(FSM) used in the Laser Beam Tracking Mechanism in the transmittermodule of each the laser communication system of the present invention.The control drive signal is simply proportional to the error signal.This is the simplest of the three algorithms. For stability reasons, theproportion or fraction of the error signal (k in the flow chart) must beless than one. For this reason, proportional control always has anoffset error, that is, the control signal always is offset from thecorrect position.

In digital control systems such as those in the tracking loop, these areall implemented by applying a set of weights to the last N errormeasurements and summing the result with the current control drivesignal (that is the integral part of the control). For example, if theweight of the current error measurement is k_(p), and all others arezero, then the algorithm is P plus I as shown in FIG. 4B. To addderivative control as shown in FIG. 4C, the weights for the last twoerror measurements would be (k_(p)+k_(d)) for the current time step and−k_(d) for the error measurement form the last time step.

This digital algorithm is very flexible and can implement many otherforms of control and filtering of the error signal. Mathematically, itcan be represented asD(t)=D(t ₁)+G*(w ₀ *E(t ₀)+w ⁻¹ *E(t ₁) +w ⁻² *E(t ₂)+. . .)

Where D is the control loop drive signal, G is the overall loop gain,w_(n) is the weight associated with time step t_(n), and E(t_(n)) is theerror signal from time step t_(n).

FIG. 4B describes the steps involved in second method of generating a“proportional plus integral” control signal for supply to the FSM usedin the Laser Beam Tracking Mechanism in the transmitter module of eachthe laser communication system of the present invention. Here, inaddition to the proportional control signal, a term that is a fraction(k_(p)) of the integral of the past error signals is added. This has theeffect of removing the offset error of proportional control alone.

FIG. 4C describes the steps involved in third method of generating a“proportional plus integral plus derivative” control signal for supplyto the FSM used in the Laser Beam Tracking Mechanism in the transmittermodule of each the laser communication system of the present invention.Here, another term may be added that is proportional to the timederivative of the error signal (multiplied by fraction k_(d) in the flowchart). The effect of this term is to provide anticipatory control, thatis, the loop uses the current derivative to predict the error in thenext time step. This can give higher loop bandwidth by reducing thelatency, but increases the noise in the loop.

In the transmitter, the goal is to keep the transmitted laser beam onthe receiver aperture. So, the null or reference point in each of theabove-described control algorithms, and the control loops which theymaintain within the transmitter, is set such that the transmitted beamis collinear with the beacon laser beam sent from the receiver to enablelaser beam pointing/tracking operations in the laser communicationsystem.

Methods of Generating a Control Signal for Supply to the Fast SteeringMirror (FSM) used in the Laser Beam Speckle Tracking Mechanism in theReceiver Module of Each the Laser Communication System of the PresentInvention

The basic control algorithms for the transmitter described in FIGS. 4Athrough 4C are similar to the control algorithms for the receiver shownin FIGS. 4D through 4F, respectively, except for the “null point” (i.e.reference point) of the control system.

In the transmitter, the goal is to keep the transmitted laser beam onthe receiver aperture. So the null or reference point in each of theabove-described control algorithms, and the control loops which theymaintain within the transmitter, is set such that the transmitted beamis collinear with the beacon laser beam sent from the receiver to enablelaser beam pointing/tracking operations in the laser communicationsystem.

Methods for Generating Spatial Phase Modulation Panel Drive Signals tobe Provided to the Spatial Phase Modulation Panel (I.E. DeformableMirror) Employed in Speckle-to-Fiber/Detector Locking Mechanism in theReceiver Module of Each Laser Communication System of the PresentInvention

The goal of the algorithms driving the Speckle-To-Fiber/Detector LockingMechanism of the present invention is to find phase and/or intensitymodulation pattern that, when added to the electric field at theentrance pupil of an optical system maximizes some metric function. Forthe case of optical communications, that metric is the time averagecommunications signal level. In general, this is an optimization problemand many of the known optimization algorithms may be applied.

Define The Problem Variables:

-   -   E(x,y); The complex e-field at the entrance pupil    -   M(x,y); Phase and/or intensity modulation applied to pupil    -   S; Time averaged signal level

For the purposes of algorithm description, it will first be assumed thatE is constant in time. Issues related to real-time application of thealgorithms will be addressed later.

In the real system, S is “calculated” by allowing the collection opticsto focus the electro-magnetic field onto the collection aperture. Thismay be simulated computationally by using standard point spread function(PSF) calculation methods to generate the spatial intensity pattern atthe focal plane. This intensity distribution may then be integrated overthe collection aperture. Typically, the PSF is calculated by taking theFourier transform of the complex e-field at the aperture (E+M). Thesquare of the amplitude of this transform then represents the intensitydistribution.

In any realization of this system, M will be in the form an N×N array ofsubregions or subapertures within the full aperture. Each of thesesubregions represents a single element or pixel of the modulationdevice. Over each subregion the value of M is constant. For such anarrangement we then have N² independent degrees of freedom for themodulator and we must find the set of N² values (Mij) that maximize S.

First Illustrative Embodiment of the Method of Present Invention, forGenerating Spatial Phase Modulation Panel Drive Signals to be Providedto the Spatial Phase Modulation Panel (I.E. Deformable Mirror) Employedin Speckle-to-Fiber/Detector Locking Mechanism in the Receiver Module ofEach Laser Communication System of the Present Invention.

FIG. 5A describes the steps involved in a first illustrative embodimentof the method of present invention, for generating spatial phasemodulation panel drive signals to be provided to the spatial phasemodulation panel (i.e. deformable mirror) employed inSpeckle-to-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention. In thismethod, the hill climbing algorithm provides a simple approach to theabove described optimization problem, wherein each degree of freedom ischanged by a small amount in sequence and the resulting effect on S ismeasured. In this way the gradient of S with respect to the N²independent variables Mij is estimated. The values of Mij are thenupdated to move in the direction of the maximum upward gradient. Thisprocess is repeated until a set of Mij is arrived at, any change fromwhich leads to a reduction in S. The algorithm has then found at least alocal maximum of S.

Second Illustrative Embodiment of the Method of Present Invention, forGenerating Spatial Phase Modulation Panel Drive Signals to be Providedto the Spatial Phase Modulation Panel (I.E. Deformable Mirror) Employedin the Speckle-to-Fiber/Detector Locking Mechanism in the ReceiverModule of Each Laser Communication System of the Present Invention

FIG. 5 describes the steps involved in a second illustrative embodimentof the method of present invention, for generating spatial phasemodulation panel drive signals to be provided to the spatial phasemodulation panel (i.e. deformable mirror) employed inSpeckle-to-Fiber/Detector Locking Mechanism in the receiver module ofeach laser communication system of the present invention. In thismethod, an alternative form of hill climbing algorithm may be used aswell. In this case, rather than testing the response of S to each M_(ij)and then calculating its updated values, the value of each M_(ij) may beupdated as soon as the response of S to that variable is determined.

Third Illustrative Embodiment of the Method of Present Invention, forGenerating Spatial Phase Modulation Drive Signals to the Supplied to theSpatial Phase Modulation Panel (I.E. Deformable Mirror) Employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the Receiver Module ofEach Laser Communication System of Present Invention

FIG. 5C describes the steps involved in a third illustrative embodimentof the method of present invention, for generating spatial phasemodulation drive signals to the supplied to the spatial phase modulationpanel (i.e. deformable mirror) employed in the Speckle-To-Fiber/DetectorLocking Mechanism in the receiver module of each laser communicationsystem of present invention. In this method, a more sophisticatedoptimization algorithm, sometimes referred to as “stochastic parallelperturbative gradient ascent”, is used. The algorithm estimates thegradient of S with respect to M_(ij) in a somewhat different way. Ateach iteration, all of the values of M_(ij) are altered in a random wayand the change in S is noted. If that change is represented by ΔS andthe perturbation applied to M by ΔM_(ij), it follows thatΔS=S(M _(ij) +ΔM _(ij))−S(M _(ij)).  (1)To find the gradient of S with respect to each element of M (M_(mn)), aTaylor series expansion of Equation 1 givesΔS/ΔM _(mn) ≈∂S/∂M _(mn) +Σ∂S/∂M _(ij) *ΔM _(ij) /ΔM _(mn) , {i≠m,j≠n}.  (2)

If the perturbations are truly stochastic and random, the second term inEquation 2 averages to zero and thus ΔS/ΔM_(mn) ≈∂S/∂M _(mn). Once thegradient of S is estimated, the values of M_(ij) may be updated to movein the direction of maximum upward gradient.

Fourth Illustrative Embodiment of the Method Of Present Invention. ForGenerating Speckle-To-Fiber/Detector Locking Drive Signals to theSpatial Phase Modulation Panel (i.e. Deformable Mirror) Employed in theReceiver Module of Each Laser Communication System of the PresentInvention

FIG. 5D describes the steps involved in a fourth illustrative embodimentof the method of present invention, for generatingspeckle-to-fiber/detector locking drive signals to the spatial phasemodulation panel (i.e. deformable mirror) employed in the receivermodule of each laser communication system of the present invention. Thisalternative method addresses a major limitation of simple gradientclimbing algorithms, namely the potential for them to locate a localmaximum of S, not the true global maximum. There are several ways toreduce the likelihood of the algorithm being trapped at a local maximum.One example is the simulated annealing algorithm. This algorithm isbased on the process that occurs when a crystalline substance is slowlycooled through its freezing point, hence the name “simulated annealing”.

The flow of the “simulated annealing” algorithm is very similar to thosedescribed above. A random perturbation of the modulator values, M_(ij),is applied and the resulting change in S is noted. If S is increased,the perturbation is accepted unconditionally. However, if S isdecreased, the perturbation may also be accepted, but with a probabilitygiven byP=Exp(ΔS/T),  (3)where T is a parameter of the algorithm. Because of the originalrelation of this algorithm to annealing, T is known as the temperatureparameter. As the algorithm searches for the maximum and the value of Sincreases, the T parameter is slowly reduced. This lowers theprobability of accepting downward changes in S. The effect of thisfinite probability of accepting negative changes in S is to provideoccasional “shocks” to the system that act to throw the search point outof local maxima. At the beginning of the process, the search point maybe far from the maximum and there may be many local maxima that couldtrap the algorithm. With a large T parameter, the probability of a shockis high, helping to prevent trapping. As the algorithm approaches thetrue maximum, the possibility of trapping is reduced, so the T parametermay be lowered.

The key to this process is the “annealing schedule”, the plan oralgorithm used to determine when and by how much to reduce T. Thisdepends intimately on the functional relation between S and M_(ij),which cannot easily be calculated since it also depends on the unknown,atmospherically affected E(x,y). Optimal annealing schedules must bedeveloped via simulation or experiment.

First Illustrative Embodiment of the Method of Present Invention, forGenerating Spatial Intensity Modulation Drive Signals to be Provided tothe Spatial Intensity Modulation Panel Employed in theSpeckle-To-Fiber/Detector Locking Mechanism in the Receiver Module ofEach Laser Communication System of the Present Invention

The basic Spatial Intensity Modulation (SIM) Control Signal Generationalgorithms described in FIGS. 6A through 6D are similar to the SPMControl Signal Generation algorithms shown in FIGS. 5A through 5D anddescribed above, respectively, except that these signals are calibratedfor and supplied to spatially intensity modulation elements, rather thanspatial phase modulation elements.

Free-Space Optical (FSO) Laser Communication System SupportingOptically-Combined Signal Transmission and Reception Channels, andEmploying Laser Beam Speckle Tracking Mechanism andSpeckle-To-Fiber/Detector Locking Mechanisms Along the Signal ReceptionChannels thereof for Automatically Stabilizing Variations in theDetected Intensity of Received Laser Beam Carrier Signals Caused byAtmospheric Turbulence Along said Signal Channels

In FIG. 7A, there is shown a second illustrative embodiment of afree-space optical (FSO) laser communication system in accordance withthe principles of the present invention, wherein first and secondcommunication terminals 11A and 11B are in communication by way of asuper broad-band FSO laser beam communication link 12 havingoptically-combined signal transmission and reception channels, andwherein Laser Beam Speckle Tracking Mechanisms andSpeckle-to-Fiber/Detector Mechanisms are employed along the signalreception channels of each communication terminal, for automaticallystabilizing variations in the detected intensity of received laser beamcarrier signals caused by atmospheric turbulence along said signalchannels. As shown in FIG. 7A, each communication terminal has atransceiver (module) which will be described in detail below.

The transceiver in each terminal includes a telescopictransmitting/receiving aperture, and an optical train embedded with thecomponents of a Laser Beam Pointing Mechanism realized by a faststeering mirror and a quad-cell detector of the kind shown in FIG. 3F,supporting optics, and a processor for carrying out one of the trackingalgorithms shown in FIGS. 4A through 4C. The transceiver furtherincludes an optical train embedded with the components of two fademitigation mechanisms of the present invention, namely: the Laser BeamSpeckle Tracking Mechanism hereof realized using a FSM, a quad-celldetector shown in FIG. 3F, supporting optics, and a processor carryingout the tracking algorithms shown in FIGS. 4D through 4F; and theSpeckle-To-Fiber/Detector Locking Mechanism hereof realized usingspatial phase or intensity phase modulator shown in FIG. 3E, a receivingfiber, a single cell detector, and the processor carrying out one of thespatial phase modulation (SPM) control signal generation algorithmsshown in FIGS. 5A through 5D.

The object of the Laser Beam Speckle Tracking Mechanism employed in boththe signal reception channels of system is to automatically track orfollow a maximum intensity laser beam speckle and move away from lowintensity (i.e. black) laser beam speckles (that might fall onto thereceiving fiber) so as to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules. Theobject of the Speckle-to-Fiber/Detector Mechanism in the signalreception channels of system is to lock a maximum intensity speckle inthe received laser beam onto the receiving fiber, so as achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the receiver modules of the system.

In this particular illustrative embodiment of the FSO communicationsystem of the present invention, the function of the tracker is now thesame on each side. This is possible because the transmitter boresightand receiver boresight are the same (defined by the single fiberlocation). If the FSM keeps the received spot on the fiber, thetransmitted beam will retrace the path of the received beam and reachthe aperture on the other side.

Free-Space Adaptive Optical (FS-OA) Laser Communication SystemSupporting Optically-Separated Signal Transmission and ReceptionChannels and Employing Laser Beam Speckle Tracking Mechanisms andSpeckle-To-Fiber/Detector Locking Mechanisms Along the Signal ReceptionChannels thereof for Automatically Stabilizing Variations in theIntensity of Received Laser Beam Carrier Signals Caused by AtmosphericTurbulence Along Said Signal Channels

A laser communications terminal with an adaptive optic compensationsystem operating should act to maximize the power collected by thecommunications fiber. However, there may be occasions when theturbulence conditions are such that the compensation system performancedegrades. This may occur when the correlation length in the atmospherebecomes less than the actuator spacing of the deformable device, meaningthat the system does not have the spatial resolution to fully correctthe wavefront, or when the strength of the aberrations exceed thedynamic range of the corrector. Whatever the cause, if the compensationsystem is operating with degraded performance it will no longer provideoptimum communications signal power. The potential exists for verysignificant variations in the received signal (fades) because the focalspot will become aberrated.

In FIGS. 8A and 8B, there is shown a third illustrative embodiment of afree-space adaptive optical (FS-AO) laser communication system 20 inaccordance with the principles of the present invention, wherein firstand second communication terminals 21A and 21B are in communication byway of a super broad-band FSO laser beam communication link havingoptically-separated signal transmission and reception channels, andwherein a standard Adaptive Optics Subsystem and the Laser Beam SpeckleTracking Mechanism and Speckle-to-Fiber/Detector Mechanism of thepresent invention are employed along the signal reception channels ofeach communication terminal, for automatically stabilizing variations inthe detected intensity of received laser beam carrier signals caused byatmospheric turbulence along said signal channels. As shown in FIG. 8A,each communication terminal 21A, 21B has a transmitter (module) 23 and areceiver (module) 24, each of which will be described in detail below.

The transmitter in each terminal includes a telescopic transmittingaperture, and an optical train embedded with the components of astandard Adaptive Optics Subsystem and the Laser Beam Pointing Mechanismrealized by a fast steering mirror and a quad-cell detector of the kindshown in FIG. 3F, supporting optics, and a processor for carrying outone of the tracking algorithms shown in FIGS. 4A through 4C.

The receiver in each terminal includes a telescopic receiving aperture,and an optical train embedded with the components of a standard AdaptiveOptics Subsystem, and the two fade mitigation mechanisms of the presentinvention, namely: the Laser Beam Speckle Tracking Mechanism hereofrealized using a FSM, a quad-cell detector shown in FIG. 3F, supportingoptics, and a processor carrying out the tracking algorithms shown inFIGS. 4D through 4F; and the Speckle-To-Fiber/Detector Locking Mechanismhereof realized using spatial phase or intensity phase modulator shownin FIG. 3E, a receiving fiber, a single cell detector, and the processorcarrying out (i) one of the spatial phase modulation (SPM) controlsignal generation algorithms shown in FIGS. 5A through 5D when usingspatial phase modulation (SPM) techniques, or (ii) one of the spatialintensity modulation (SIM) control signal generation algorithms shown inFIGS. 6A through 6D when using spatial phase modulation (SPM)techniques.

The object of the standard Adaptive Optics Subsystem is to remove phaseaberrations in the wavefront of the transmitted and received laser beamto that it is focused to its diffraction limit at the entrance pupil ofthe receiver of the communication terminal.

The object of the Laser Beam Speckle Tracking Mechanism employed in boththe signal reception channels of system is to automatically track orfollow a maximum intensity laser beam speckle and move away from lowintensity (i.e. black) laser beam speckles (that might fall onto thereceiving fiber) so as to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules.

The object of the Speckle-to-Fiber/Detector Mechanism in the signalreception channels of system is to lock a maximum intensity speckle inthe received laser beam onto the receiving fiber, so as achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the receiver modules of the system.

Free-Space Adaptive Optical (FS-OA) Laser Communication SystemSupporting Optically-Combined Signal Transmission and Reception Channelsand Employing Laser Beam Speckle Tracking Mechanisms andSpeckle-To-Fiber/Detector Mechanisms Along the Signal Reception Channelsthereof for Automatically Stabilizing Variations in the Intensity ofReceived Laser Beam Carrier Signals Caused by Atmospheric TurbulenceAlong Said Signal Channels

In FIGS. 9A and 9B, there is shown a fourth illustrative embodiment of afree-space adaptive optical (FS-AO) laser communication system 30 inaccordance with the principles of the present invention, wherein firstand second communication terminals 31A and 31B are in communication byway of a super broad-band FSO laser beam communication link 32 havingoptically-combined signal transmission and reception channels, andwherein a standard Adaptive Optics Subsystem and the Laser Beam SpeckleTracking Mechanism and Speckle-to-Fiber/Detector Mechanism of thepresent invention are employed along the signal reception channels ofeach communication terminal, for automatically stabilizing variations inthe detected intensity of received laser beam carrier signals caused byatmospheric turbulence along said signal channels. As shown in FIGS. 9Aand 9B, each communication terminal has a transceiver (module) 31A whichwill be described in detail below.

The transceiver in each terminal includes a telescopictransmitting/receiving aperture, and an optical train embedded with thecomponents of a standard Adaptive Optics Subsystem and the Laser BeamPointing Mechanism realized by a fast steering mirror and a quad-celldetector of the kind shown in FIG. 3F, supporting optics, and aprocessor for carrying out one of the tracking algorithms shown in FIGS.4A through 4C. The optical train of the transceiver further includes thecomponents of the two fade mitigation mechanisms of the presentinvention, namely: the Laser Beam Speckle Tracking Mechanism hereofrealized using a FSM, a quad-cell detector shown in FIG. 3F, supportingoptics, and a processor carrying out the tracking algorithms shown inFIGS. 4D through 4F; and the Speckle-To-Fiber/Detector Locking Mechanismhereof realized using spatial phase or intensity phase modulator shownin FIG. 3E, a receiving fiber, a single cell detector, and the processorcarrying out (i) one of the spatial phase modulation (SPM) controlsignal generation algorithms shown in FIGS. 5A through 5D when usingspatial phase modulation (SPM) techniques, or (ii) one of the spatialintensity modulation (SIM) control signal generation algorithms shown inFIGS. 6A through 6D when using spatial phase modulation (SPM)techniques.

The object of the standard Adaptive Optics Subsystem is to remove phaseaberrations in the wavefront of the transmitted and received laser beamso that it is focused to its diffraction limit at the entrance pupil ofthe receiver of the communication terminal.

The object of the Laser Beam Speckle Tracking Mechanism employed in boththe signal reception channels of system is to automatically track orfollow a maximum intensity laser beam speckle and move away from lowintensity (i.e. black) laser beam speckles (that might fall onto thereceiving fiber) so as to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules.

The object of the Speckle-to-Fiber/Detector Mechanism in the signalreception channels of system is to lock a maximum intensity speckle inthe received laser beam onto the receiving fiber, so as achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the receiver modules of the system.

FIG. 9E shows a tripod-mounted FS-OA laser communication terminalconstructed in accordance with the architecture of FIG. 9B, wherein thetransceiver telescope has a 20 cm aperture and is mounted beneath theoptical bench of the terminal, and a high-spatial resolution AO(WFS/WFC) compensation subsystem is provided for communication linkcharacterization/compensation, and a real-time subsystem is provided forstabilizing the intensity of laser beam carrier signals detected at thetransceiver of the system.

FIG. 9F shows a compact FS-OA laser communication terminal alsoconstructed in accordance with the architecture of FIG. 9B, comprising:a transceiver telescope having a 15 cm aperture and mounted beneath theoptical bench of the terminal; a high-spatial resolution AO (WFS/WFC)compensation subsystem with communication linkcharacterization/compensation; a real-time subsystem having a Laser BeamSpeckle Tracking Mechanism (including a fast steering mirror FSM)); anda Speckle-to-Fiber/Detector Locking Mechanism (including a deformablemirror as a spatial phase modulation panel) for stabilizing theintensity of laser beam carrier signals detected at the transceiver ofthe system, in response to atmospheric turbulence.

Free-Space Adaptive Optical (FS-OA) Laser Communication SystemSupporting Optically-Combined Signal Transmission and Reception Channelsand Employing Speckle-To-Receiver-Aperture Locking Mechanism RequiringReceiver-To-Transmitter “Figure of Merit” Communication Feedback, andLaser Beam Speckle Tracking Mechanisms and Speckle-To-Fiber/DetectorLocking Mechanisms Along the Signal Reception Channels thereof forAutomatically Stabilizing Variations in the Intensity of Received LaserBeam Carrier Signals Caused by Atmospheric Turbulence Along Said SignalChannels.

In FIGS. 10A and 10B, there is shown a fifth illustrative embodiment ofa free-space optical (FSO) laser communication system 40 in accordancewith the principles of the present invention, wherein first and secondcommunication terminals 41A and 41B are in communication by way of asuper broad-band FSO laser beam communication link 42 havingoptically-separated signal transmission and reception channels, andwherein the Speckle-To-Receiver-Aperture Locking Mechanism (requiringreceiver-to-transmitter “figure of merit” communication feedback) of thepresent invention is employed along the transmission channels of eachcommunication terminal, and the Laser Beam Speckle Tracking Mechanismand Speckle-to-Fiber/Detector Mechanism of the present invention areemployed along the signal reception channels of each communicationterminal, for automatically stabilizing variations in the detectedintensity of received laser beam carrier signals caused by atmosphericturbulence along said signal channels. As shown in FIG. 10B, eachcommunication terminal has a transmitter (module) 41A and a receiver(module) 41B, each of which will be described in detail below.

The transmitter in each terminal includes a telescopic transmittingaperture, and an optical train embedded with the components of theSpeckle-to-Receiver-Aperture Locking Mechanism realized by a faststeering mirror and a quad-cell detector of the kind shown in FIG. 3F,supporting optics, and a processor for carrying out one of the trackingalgorithms shown in FIGS. 4A through 4C using the received communicationlink power signal (sent via the laser communication link), as shown inFIG. 10B and described in detail hereinabove with reference to thedescription of the corresponding Speckle-to-Receiver-Aperture LockingMethod.

The receiver in each terminal includes a telescopic receiving aperture,and an optical train embedded with the components of the two fademitigation mechanisms of the present invention, namely: the Laser BeamSpeckle Tracking Mechanism hereof realized using a FSM, a quad-celldetector shown in FIG. 3F, supporting optics, and a processor carryingout the tracking algorithms shown in FIGS. 4D through 4F; and theSpeckle-To-Fiber/Detector Locking Mechanism hereof realized usingspatial phase or intensity phase modulator shown in FIG. 3E, a receivingfiber, a single cell detector, and the processor carrying out (i) one ofthe spatial phase modulation (SPM) control signal generation algorithmsshown in FIGS. 5A through 5D when using spatial phase modulation (SPM)techniques, or (ii) one of the spatial intensity modulation (SIM)control signal generation algorithms shown in FIGS. 6A through 6D whenusing spatial phase modulation (SPM) techniques.

The object of the Laser Beam Speckle Tracking Mechanism employed in boththe signal reception channels of system is to automatically track orfollow a maximum intensity laser beam speckle and move away from lowintensity (i.e. black) laser beam speckles (that might fall onto thereceiving fiber) so as to achieve a first level of optical signalintensity stabilization at signal detector of the receiver modules.

The object of the Speckle-to-Fiber/Detector Mechanism in the signalreception channels of system is to lock a maximum intensity speckle inthe received laser beam onto the receiving fiber, so as achieve a secondlevel of optical signal intensity stabilization at the signal detectorin the receiver modules of the system.

This FSO laser communication system shown in FIGS. 10A and 10B issimilar to the laser communication system shown in FIGS. 3A and 3B, withthe addition of Speckle-To-Receiver-Aperture Locking Mechanism. Asdescribed hereinabove, this is essentially the same basic technique asthe Speckle-To-Fiber/Detector Locking Method of the present invention.In this case, however, a spatial phase or intensity modulator is used inthe transmitter to optimize the laser beam signal passed into thereceiver aperture on the other side of the communication link. Toachieve this, it is necessary for the receiver to send the communicationlink power signal that it is using to perform theSpeckle-to-Fiber/Detector Locking Method back to the transmitter via theoptical or other communications link. The transmitter uses this signalas the figure of merit in the algorithm that drives its spatial phase orintensity modulator. This control loop acts to maintain (i.e. lock) abright speckle in the transmitted laser beam onto the receiver aperture,while the receiver loop maintains a bright speckle in the received laserbeam in its onto the receiver fiber (or detector).

It is understood that the image processing technology employed insystems of the illustrative embodiments may be modified in a variety ofways which will become readily apparent to those skilled in the art andhaving the benefit of the novel teachings disclosed herein. All suchmodifications and variations of the illustrative embodiments thereofshall be deemed to be within the scope and spirit of the presentinvention as defined by the Claims to Invention appended hereto.

1. A free-space optical (FSO) laser communication system automaticallystabilizing variations in the detected intensity of laser beam carriersignals, caused by atmospheric turbulence along signal receptionchannels supported within said FSO laser communication system, said FSOlaser communication system comprising: at least first and secondcommunication terminals in optical communication by way of a broad-bandFSO laser beam communication link supporting signal transmission andreception channels; wherein each said communication terminal has atransmitter module and a receiver module; and wherein each said receivermodule includes a receiving aperture for receiving a FSO laser beamcarrier signal transmitted from said transmitter module of another oneof said communication terminals; a fast steering mirror (FSM) forsteering said FSO laser beam carrier signal along an optical pathwayhaving downstream direction; a beam splitter, disposed downstream fromsaid FSM, for splitting said FSO laser beam carrier signal into a firstsignal component and a second signal component; a receiving opticalfiber, disposed at the end of said optical pathway, for receiving thesecond component of said FSO laser beam carrier signal aftertransmission along said optical pathway; a single-cell signal detectorin optical communication with said receiving optical fiber, fordetecting the intensity of said second component of said FSO laser beamcarrier signal and generating an electrical signal correspondingthereto; a multi-segment signal detector, disposed downstream from saidbeam splitter, for detecting the intensity of said first signalcomponent of said FSO laser beam carrier signal, and generatingelectrical signals corresponding thereto; a processor for automaticallyanalyzing signals generated from said multi-segment signal detector,controlling said FSM, and automatically tracking or following a maximumintensity laser beam speckle in said FSO laser beam carrier signal, andmoving low intensity laser beam speckles appearing in said FSO laserbeam carrier signal away from said receiving optical fiber, and therebyachieving a first level of optical signal intensity stabilization atsaid single-cell signal detector in said receiver module; a spatialmodulator for spatially modulating said second component of said FSOlaser beam carrier signal; said processor further analyzing electricalsignals produced by said single-cell signal detector, controlling saidspatial modulator, and spatially modulating said second component ofsaid FSO laser beam carrier signal so as to lock a maximum intensityspeckle appearing in the received second component of said FSO laserbeam carrier signal, onto said receiving optical fiber, and therebyachieving a second level of optical signal intensity stabilization atsaid single-cell signal detector in said receiver module.
 2. The FSOlaser communication system of claim 1, wherein said signal transmissionand reception channels are optically-separated.
 3. The FSO lasercommunication system of claim 1, wherein said signal transmission andreception channels are optically-combined.
 4. The FSO lasercommunication system of claim 1, wherein said receiving aperturecomprises a telescopic receiving aperture.
 5. The FSO lasercommunication system of claim 1, wherein said transmitter module andsaid receiver module are realized as separate modules.
 6. The FSO lasercommunication system of claim 1, wherein said transmitter module andsaid receiver module are realized as a single transceiver module.
 7. TheFSO laser communication system of claim 1, wherein said spatialmodulator is a spatial phase modulator.
 8. The FSO laser communicationsystem of claim 7, wherein said spatial phase modulator is realized as aspatial phase modulation panel having a plurality spatial phasemodulation elements.
 9. The FSO laser communication system of claim 8,wherein said spatial phase modulation panel comprises a deformablemirror.
 10. The FSO laser communication system of claim 1, wherein saidspatial modulator is a spatial intensity modulator.
 11. The FSO lasercommunication system of claim 10, wherein said spatial intensitymodulator is realized as a spatial intensity modulation panel having aplurality spatial intensity modulation elements.
 12. The FSO lasercommunication system of claim 1, wherein said multi-segment detector isa quad-cell detector.
 13. The FSO laser communication system of claim 8,wherein said processor generates spatial phase modulation (SPM) controlsignals for controlling said spatial phase modulation panel, so as tolock said maximum intensity speckle in the received laser beam carriersignal onto said receiving optical fiber, and achieve said second levelof optical signal intensity stabilization at said single-cell signaldetector in said receiver module.
 14. The FSO laser communication systemof claim 11, wherein said processor generates spatial intensitymodulation (SIM) control signals for controlling said spatial intensitymodulation panel, so as to lock said maximum intensity speckle in thereceived laser beam carrier signal onto said receiving optical fiber,and achieve said second level of optical signal intensity stabilizationat said single-cell signal detector in said receiver module.
 15. Amethod of automatically stabilizing variations in the detected intensityof free-space optical (FSO) laser beam carrier signals caused byatmospheric turbulence in a free-space optical (FSO) laser communicationsystem having at least first and second communication terminals inoptical communication by way of a broad-band FSO laser beamcommunication link supporting signal transmission and receptionchannels, wherein each said transmission and reception channel has atransmitter module and a receiver module, and wherein each said receivermodule includes (i) a receiving aperture for receiving a FSO laser beamcarrier signal transmitted from said transmitter module of another oneof said communication terminals, along a optical pathway havingdownstream direction, (ii) a receiving optical fiber, disposed at theend of said optical pathway, for receiving said FSO laser beam carriersignal after transmission along said optical pathway, and (iii) asingle-cell signal detector in optical communication with said receivingoptical fiber, for detecting the intensity of said FSO laser beamcarrier signal and generating an electrical signal correspondingthereto, said method comprising the steps of: (a) providing within eachsaid receiver module, a fast steering mirror (FSM), disposed upstreamfrom said receiving optical fibers, for steering said FSO laser beamcarrier signal along an optical pathway having a downstream direction;(b) providing a beam splitter, disposed downstream from said FSM, andsplitting said FSO laser beam carrier signal into a first signalcomponent and a second signal component; (c) providing a multi-segmentsignal detector, disposed downstream from said beam splitter, anddetecting the intensity of said first signal component of said FSO laserbeam carrier signal, and generating electrical signals correspondingthereto; (d) processing signals generated from said multi-segment signaldetector, controlling said FSM, and automatically tracking or followinga maximum intensity laser beam speckle in said FSO laser beam carriersignal, and moving low intensity laser beam speckles appearing in saidFSO laser beam carrier signal away from said receiving optical fiber,and thereby achieving a first level of optical signal intensitystabilization at said single-cell signal detector in said receivermodule; (e) providing a spatial modulator, disposed downstream from saidbeam splitter, and spatially modulating said second component of saidFSO laser beam carrier signal; and (f) further processing electricalsignals produced by said single-cell signal detector, controlling saidspatial modulator, and spatially modulating said second component ofsaid FSO laser beam carrier signal so as to lock a maximum intensityspeckle appearing in the received second component of said FSO laserbeam carrier signal, onto said receiving optical fiber, and therebyachieving a second level of optical signal intensity stabilization atsaid single-cell signal detector in said receiver module.
 16. The methodof claim 15, wherein said signal transmission and reception channels areoptically-separated.
 17. The method of claim 15, wherein said signaltransmission and reception channels are optically-combined.
 18. Themethod of claim 15, wherein said receiving aperture comprises atelescopic receiving aperture.
 19. The method of claim 15, wherein saidtransmitter module and said receiver module are realized as separatemodules.
 20. The method of claim 15, wherein said transmitter module andsaid receiver module are realized as a single transceiver module. 21.The method of claim 15, wherein in step (e), said spatial modulator is aspatial phase modulator.
 22. The method of claim 21, wherein in step(e), said spatial phase modulator is realized as a spatial phasemodulation panel having a plurality spatial phase modulation elements.23. The method of claim 8, wherein in step (e), said spatial phasemodulation panel comprises a deformable mirror.
 24. The method of claim1, wherein in step (e), said spatial modulator is a spatial intensitymodulator.
 25. The method of claim 24, wherein in step (e), said spatialintensity modulator is realized as a spatial intensity modulation panelhaving a plurality spatial intensity modulation elements.
 26. The methodof claim 15, wherein in step (c), said multi-segment detector is aquad-cell detector.
 27. The method of claim 22, wherein in step (e),said processor generates spatial phase modulation (SPM) control signalsfor controlling said spatial phase modulation panel, so as to lock saidmaximum intensity speckle in the received laser beam carrier signal ontosaid receiving optical fiber, and achieve said second level of opticalsignal intensity stabilization at said single-cell signal detector insaid receiver module.
 28. The method of claim 25, wherein in step (e),said processor generates spatial intensity modulation (SIM) controlsignals for controlling said spatial intensity modulation panel, so asto lock said maximum intensity speckle in the received laser beamcarrier signal onto said receiving optical fiber, and achieve saidsecond level of optical signal intensity stabilization at saidsingle-cell signal detector in said receiver module.